The security agency is believed to routinely keep files on people of interest, including political activists and campaigners. Archived documents have revealed Mr Corbyn attended several events supporting the militant Republicans during the 1980s and 1990s.
An unnamed source reportedly told The Telegraph: “If there was a file on someone, it meant they had come to notice. We opened a temporary file and did a preliminary investigation. It was then decided whether we should open a permanent file on them.”
A spokesman for the Home Office said they could neither confirm nor deny who M15 kept files on in the interests of national security.
The Metropolitan Police’s Special Branch reportedly monitored Mr Corbyn at around the same time because of his involvement with anti-racist groups, but it is unclear whether intelligence was shared between the organisations. The MI5 file is reported to have been opened “by the early 1990s”.
A spokesman for Mr Corbyn said: “M15 kept files on many peace and Labour movement campaigners at the time, including anti-Apartheid activists and trade unionists.”
M15 has previously come under fire from privacy organisations for keeping files on leading left-wing historians and academics, including Eric Hobsbawm and Christopher Hill, who were both once members of the Communist Party.
Nobel prize-winning author Doris Lessing was also spied on by the security service for 20 years – long after she had abandoned her youthful interest in communism.
At the height of the troubles, Mr Corbyn was a backbench MP for Islington North and appeared at a number of Republican protest events, along with his long-time friend, the now Shadow Chancellor John McDonnell.
Mr Corbyn was arrested in 1986 for joining a protest outside the Old Bailey intended to show solidarity with members of…