MI5 opened a file on Jeremy Corbyn because of his links to the IRA, according to reports.
The UK's Labour leader, who has claimed his efforts to "reach out" to republicans were an effort to bring about peace in the North, was investigated over fears he could have been a threat to national security at a time when he was supporting convicted terrorists and campaigning for a united Ireland, The Daily Telegraph reported.
A spokesman for the Labour leader said the British Security Service kept files on "many peace and labour movement campaigners" at the time.
Mr Corbyn's association with prominent republicans - including members of the IRA - is well known and the newspaper reported a file on him was opened by MI5 by the early 1990s.
A source close to the investigation 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 file would be opened on "someone who sympathises with a certain group, or is friends with a specific person" and the purpose was to "assess whether the person was a threat", the source added.
Both Mr Corbyn and shadow chancellor John McDonnell have faced scrutiny over their association with the IRA.
On Friday, Mr McDonnell apologised again for comments he made praising the IRA's "bravery" in 2003.
After becoming Labour leader in 2015, Mr Corbyn defended reaching out to republicans at the height of the Troubles - including inviting IRA terrorists to the Commons - insisting he "wanted the violence to stop".
The Labour leader said: "I invited people who were former prisoners, who had served their sentences, into the House of Commons at that time for a discussion about the prison situation and conditions in Northern Ireland, for a discussion about the possibility of a political development and a political solution.
"All through the 1980s, yes, I did make myself very unpopular with some people by a preparedness to reach out to the republican tradition in Ireland, to say ultimately this war is unwinnable by either side."
He added: "I don't want violence, I don't want killing, I don't want all the horrors that go with it."
It has previously been claimed that the Metropolitan Police special branch had opened a file on Mr Corbyn over his association with anti-racism campaigns, allegations which are being investigated by Christopher Pitchford's undercover policing inquiry.
A spokesman for the Labour leader said: "MI5 kept files on many peace and labour movement campaigners at the time, including anti-Apartheid activists and trade unionists.
"The Pitchford Inquiry is currently investigating how the Security Services carried out surveillance of political figures such as former foreign secretary Jack Straw, former Northern Ireland secretary Peter Hain and Jeremy Corbyn."
A Telegraph investigation into Mr Corbyn's association with IRA figures revealed his support for Hugh Doherty, a member of the IRA's Balcombe Street gang which carried out a wave of attacks in south-east England in the 1970s.
The newspaper reported that in 1987 Mr Corbyn handed a petition to then-prime minister Margaret Thatcher demanding better visiting conditions for Doherty and his fellow IRA prisoner Nat Vella and "the immediate transfer of Irish political prisoners to prisons near their homes".
Mr Corbyn also supported the cause of IRA bombmaker Dessie Ellis, who was jailed for 10 years in Dublin in the 1980s but later went on to become a member of the Dáil.