Senior Tories have blasted the left winger as a “terrorist sympathiser” over allegations the nation’s domestic counter-intelligence and security agency had looked into Mr Corbyn’s activities.
A source close to the investigation of Mr Corbyn told the Telegraph MI5 had officially opened a file on the current Labour leader by the early 1990’s.
An MI5 source said: “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.”
The source explained a file would be opened on “someone who sympathises with a certain group, or is friends with a specific person” so they could then “assess whether the person was a threat”.
It was claimed last year the Metropolitan Police’s Special Branch was also looking at the then backbench MP – although it is unclear if their investigation was linked with MI5.
A spokesman for Mr Corbyn said: “MI5 kept files on many peace and Labour movement campaigners at the time, including anti-Apartheid activists and trade unionists.”
The Labour leader, who was criticised for describing members of Palestinian terror group Hamas his “friends”, regularly attend Republican protest events during the height of the Troubles.
And a freshly surfaced interview with John McDonnell has revealed he and Mr Corbyn would strongly lobby on behalf of Irish Republicans in Parliament, with the shadow Chancellor bragging they “pinned people up against the wall and said ‘do you know what has happened in Ireland in your name?’”
Conservative peer Lord Tebbit, whose wife was left permanently disabled by the IRA’s 1984 Grand Hotel bombing in Brighton, blasted the beleaguered Labour leader as a “terrorist sympathiser”.
He said: “I think it just marks him out as what he clearly is: a hard-Left, Marxist, terrorist sympathiser.
“It leaves him very comfortable in the company of McDonnell, who, as we know, said that they will gain power not through Parliament but on the streets and the picket lines.”
“It’s a comfort to know that they were indeed being monitored, although clearly they stayed just on the right side, for them, of being charged with any offences. It just makes me feel that Mrs May is being perhaps unnecessarily indulgent towards the Labour party in order to win this election.”
A spokesman for Mr Corbyn said: “Jeremy campaigned for peace in Northern Ireland.
“To do so, he campaigned for the rights of all to be respected and spoke to people on all sides of the conflict.
“Jeremy campaigned for fair trials and against miscarriages of justice, after a series of well publicised cases, such as the Guildford Four and the Birmingham Six.”
The spokesman added: “John and Jeremy do not pin people to walls. This is clearly figurative language.”