At the beginning of 1914 the British Army had a reported strength of 710,000 men including reserves, of which around 80,000 were regular troops ready for war. By the end of the First World War almost 1 in 4 of the total male population of the United Kingdom of Great Britain and Ireland had joined up, over five million men. Of these, 2.67 million joined as volunteers and 2.77 million as conscripts (although some volunteered after conscription was introduced and would most likely have been conscripted anyway). Monthly recruiting rates for the army varied dramatically.
At the outbreak of war on 4 August 1914, the British regular army numbered 247,432 serving officers and other ranks. This did not include reservists liable to be recalled to the colours upon general mobilization or the part-time volunteers of the Territorial Army. About one-third of the peace-time regulars were stationed in India and were not immediately available for service in Europe.
For a century British governmental policy and public opinion were against conscription for foreign wars. At the start of World War I the British Army consisted of six infantry divisions, one cavalry division in the United Kingdom formed after the outbreak of war, and four divisions overseas. Fourteen Territorial Force divisions also existed, and 300,000 soldiers in the Reserve Army. Lord Kitchener, the Secretary of State for War, considered the Territorial Force untrained and useless. He believed that the regular army must not be wasted in immediate battle, but instead used to help train a new army with 70 divisions—the size of the French and German armies—that he foresaw would be needed to fight a war lasting many years.
In 1914, the British had about 5.5 million men of military age, with another 500,000 reaching the age of 18 each year. The initial call for 100,000 volunteers was far exceeded, almost half a million men enlisted in two months (see the graph). Around 250,000 underage boys also volunteered, either by lying about their age or giving false names, these were always rejected if the lie was discovered.
One early peculiarity was the formation of 'pals battalions': groups of men from the same factory, football team, bank or similar, joining and fighting together. The idea was first suggested at a public meeting by Lord Derby. Within three days, he had enough volunteers for three battalions. Lord Kitchener gave official approval for the measure almost instantly and the response was impressive. Manchester raised nine specific pals battalions (plus three reserve battalions); one of the smallest was Accrington, in Lancashire, which raised one. One consequence of pals battalions was that a whole town could lose its military-aged menfolk in a single day of battle.
The women's suffrage movement was sharply divided, the slight majority becoming very enthusiastic patriots and asking their members to give white feathers (the sign of the coward) in the streets to men who appeared to be of military age to shame them into service. After assaults became prevalent the Silver War Badge was issued to men who were not eligible or had been medically discharged.
The popular music hall artistes of the time worked enthusiastically for recruitment. Harry Lauder toured the music halls, recruiting young soldiers on stage in front of the audience, often offering 'ten pounds for the first recruit tonight'. Marie Lloyd sang a recruiting song I didn't like you much before you joined the army, John, but I do like you, cockie, now you've got yer khaki on (1914). Vesta Tilley sang The Army of Today's alright.
The available pool was diminished by roughly 1.5 million men who were "starred": kept in essential occupations. Almost 40 percent of the volunteers were rejected for medical reasons. Malnutrition was widespread in U.K. society; working class 15‑year‑olds had an average height of 5 feet 2 inches (160cm) while the upper class was 5 feet 6 inches (171cm) 
From the start Kitchener insisted that the British must build a massive army for a long war, arguing that the British were obliged to mobilise to the same extent as the French and Germans. His goal was 70 divisions and the Adjutant General asked for 92,000 recruits per month, well above the number volunteering (see the graph). The obvious remedy was conscription, which was a hotly debated political issue. Many Liberals, including Prime Minister H. H. Asquith, were ideologically opposed to compulsion, but by 1915 they were wavering.
15 July 1915 the National Registration Act was passed by the British Government, exactly one month later there was a census of every citizen aged 15 to 65, approximately 29 million forms completed.  A pink coloured registration card was issued to each registree, listing pertinent data. These records were ordered to be destroyed in 1921, yet fortunately some did survive, including the recently discovered forms of 2,409 women living in Cirencester, now[when?] transcribed and made publicly available. 
The Cabinet were given a strong warning in September 1915 in a paper presented by their only Labour member, Minister of Education Arthur Henderson. He argued many working men would strongly resist serving a nation in which they did not have a legitimate share in governing. They resented the idea of being dragooned to face possible death by a Parliament they had no part in electing—forty percent of men over 21 were denied the vote by the franchise and registration laws (until the Representation of the People Act of 1918). To them conscription was yet another theft of working men's rights by rich capitalists. The only counter-argument the Government could offer was it was of absolute necessity. Workers must be convinced that there were too few volunteers to meet the need, meaning the loss of the war and the end of Britain!; Army leaders should convince Union leaders by stating military facts. Henderson's own son was killed in the war a year later. 
Main article: Derby Scheme
The Derby Scheme was launched in Autumn 1915 by the Earl of Derby, Kitchener's new Director General of Recruiting, to determine how many new recruits could be signed up, using appointed canvassers visiting eligible men at home to persuade them to 'volunteer' for war service.  
The scheme announcement caused an initial rise in recruitment, as some preferred to go to the recruiting office rather than wait for the inevitable. The process began with each eligible man’s registry card, from the August 1915 National Registry, being copied onto another card which was sent to his local constituency's Parliamentary recruiting committee. This Committee appointed 'canvassers' who they considered "tactful and influential men", and not themselves liable for service, to visit the men in their homes.  Many canvassers were experienced in politics, though discharged veterans and the fathers of serving soldiers proved the most effective, while some just used threats to persuade. Although women were not allowed to canvas, they did contribute by tracking men who had moved address. 
Every man would be given a copy of a letter from the Earl of Derby, explaining the programme and, rather dramatically, stating that they were in "a country fighting, as ours is, for its very existence".  and had to state whether or not he was willing to attest to enlist.
Those who did agree to attest had to promise to present themselves at their recruiting office within 48 hours, while some were accompanied there immediately. If found they passed a medical they were sworn in and paid a 'signing bonus' of 2s 9d. The next day they were transferred to Army Reserve B. A khaki armband bearing the Royal Crown  was provided to all who had enlisted, or officially rejected, as well as to starred and discharged men. (This ceased once conscription was introduced, Jan.1916).
The enlistee’s data was copied onto another card and used to assign him to one of 46 married or unmarried age groups, and promised that only entire groups would be called for active service, after 14 days prior notice. Single men's groups would be called before married, any who wed after the day the scheme began were still classified as single. Married men were assured their group would not be called if too few single men attested – unless conscription was introduced.
The scheme was undertaken during November – December 1915 and obtained 318,553 medically fit single men. However, 38% of single men and 54% of married men had resisted the orchestrated pressure to enlist in the war, so the British Government, determined to ensure a supply of replacements for mounting casualties overseas, instead passed The Military Service Act of compulsory conscription, 27 January 1916. 
As there were too few volunteers to fill the ranks, the Military Service Bill was introduced in January 1916; compulsory conscription for the first time in Britain's history. Every unmarried man and childless widower between 18 and 41 was offered three choices:
In May 1916 the bill was extended to married men and in April 1918 the upper age was raised to 50 (or 56 if need arose). Ireland was excluded due to the 1916 Easter Uprising, yet many Irish men still volunteered to enlist. 
One of the immediate effects of conscription was the biggest protest demonstration Britain had seen since the 1914 anti-war protests. Thousands marched from London’s East End to Trafalgar Square, where they listened to speeches from Sylvia Pankhurst and a Glasgow councillor who reported that engineers had come out on strike along the Clyde. The event ended in violence when a group of soldiers and sailors charged the crowd, ripped their banners and threw dye in their faces. Some of the assailants were arrested, but Bow Street magistrates let them off with nominal fines and a warning to “leave these idiots alone in future”.
As Arthur Henderson had warned, compulsion was unpopular, by July 1916, 93,000 (30%) of those called up for military service had failed to appear (In reality they had little chance of escape, although a lucky few were hidden by sympathisers)  and 748,587 had claimed an exemption – the men, or their employers, could appeal to a civilian Military Service Tribunal in their district on the grounds of work of national importance, business or domestic hardship, medical unfitness or conscientious objection. Dealing with these cases filled the judicial courts, although the standards of appeal tribunals were dubious: in York a case was determined in an average of eleven minutes, while two minutes was typical at Paddington, London. In addition to those objecting to enlisting, there were 1,433,827 already starred as being in a war occupation, ill or already discharged from service due to illness; the enforced conscription act ultimately failed to satisfy Govt demand for lives. 
Although it has been the focus of the tribunals' image since the war, only two percent of those appealing were conscientious objectors. Around 7,000 of them were granted non-combatant duties, while a further 3,000 ended up in special work camps. 6,000 were imprisoned. Forty two were sent to France to potentially face a firing squad. Thirty five were formally sentenced to death, but immediately reprieved, with ten years penal servitude substituted.
Many of those appealing were given some kind of exemption, usually temporary (between a few weeks and six months) or conditional on their situation at work or home remaining serious enough to warrant their staying at home. In October 1916 1.12 million men held tribunal exemption or had cases pending, by May 1917 this had fallen to 780,000 exempt and 110,000 pending. At this point 1.8 million men were exempted, more men than were serving overseas with the British Army. Some men were exempted on the condition that they joined the Volunteer Training Corps for part-time training and home defence duties; by February 1918, 101,000 men had been directed to the Corps by the tribunals.
A newspaper report on 17 February 1916 listed five sample appeals for exemption from conscription, and their results.
|Appealer(s)||On behalf of||Plea||Result of plea|
|a firm of art dealers||their clerk||busy with a big export trade||appeal rejected|
|Royal Academy||W. R. H. Lamb (its secretary)||busy with 12,000 incoming works; no available replacement||his conscription was delayed 3 months|
|a tobacco and cigarette manufacturers||their blender||"we supply army officers"||appeal rejected|
|Oswald Stoll||J. P. Seaborn (artist and photographer)||busy making images, etc. for Mr. Stoll's theatres||wait while accumulating a list of about 40 appealed employees|
|Mr. Brenman (clog dancer, member of the music hall act Five Dorinos)||needed to hold group together||his conscription was delayed 1 month|
In 1917 the Commons passed a bill to give the vote to all males over 21 (and to many women as well), but it did not become law until 6 February 1918. In the same month occupational exemptions were withdrawn for men 18 to 23, in October 1918 there were 2,574,860 men in reserved occupations. Men aged 18½ were sent to the fronts starting in March 1918, violating a pledge to keep them safe until 19. There were also problems with men’s suitability for active service. The healthy manpower was simply not there – recruits were sorted into three classes, A, B, and C, in order of fitness for front-line service. In 1918, 75 percent were classed as A, some B men also had to be sent into the trenches.
The upper limit on the number of men conscripted is usually calculated by assuming that all recruits after 1 March 1916 were conscripts: 1,542,807 men, 43.7% of those who served in the Army during the war. However, Derby had enlisted 318,553 single men in Special Reserve B, who were called up in spring 1916, which reduces the conscripted to 37%. The married men who had attested in the Derby plan are harder to categorize because they were not called up from the Reserve but swept up with the rest. It seems that somewhat less than 35% of the men in the army were compelled to serve.
In 1918, legislation intended to extend conscription to Ireland was opposed by a broad movement of Irish nationalists and the Catholic Church in Ireland. The central role of Sinn Féin in this movement boosted its popularity in the buildup to the November general election.
Official advertising for recruits was in the control of the Parliamentary Recruiting Committee (PRC), but private efforts such as that by the London Opinion with the now iconic image of Kitchener were also produced. Posters were prepared in different sizes according to where they would be used; two sizes 40 by 30 in (1,020 by 760 mm) and 20 by 15 in (510 by 380 mm) were common. The PRC was formed of the Prime Minister and heads of political parties later joined by other representatives such as for the Trades Union Congress; its sub-committees handled the preparation and publishing of recruiting materials.
Over the course of the war the PRC was responsible for 164 poster designs and 65 pamphlets. Between those and other advertising materials a total of over 54 million pieces were produced at a cost of £40,000. According to numbers of posters produced, the most popular official PRC posters were:
The "King's Message" reprinted in poster form ran to 290,000 copies.
Notable men who served as army recruiters during the war included: