In the United Kingdom, the hard left are the left-wing political movements and ideas outside the mainstream centre-left.
The term was first used in the context of debates within both the Labour Party and the broader left in the 1980s to describe Trotskyist groups such as the Militant tendency, Socialist Organiser and Socialist Action. Within the party, the "hard left", represented by the Campaign Group, subscribed to more strongly socialist views while the "soft left", associated for example with the Tribune Group, embraced more moderate social democratic ideas.
Politicians commonly described as being on the hard left of the Labour Party at the time included Tony Benn, Derek Hatton, Ken Livingstone, Dennis Skinner, Jeremy Corbyn and Eric Heffer.
The term has since then often been used pejoratively by Labour's political opponents, for example, during the Conservative Party's election campaigns of the early 1990s, and by the media. 
Condemnation by label is a favourite tactic of political antagonism ... Descriptions like 'hard left', 'far left' ... all have extra connotations, political under-meanings to damage the people they describe
Key words and phrases like 'hard left', 'extremist' and 'Soviet style' are explicitly derogatory and dismissive labels which mask a serious lack of information and analysis about the theory and practice of socialism and communism.