MI5 opened a file on Jeremy Corbyn because of his links to IRA terrorists, according to reports.
The Labour leader, who has claimed his efforts to "reach out" to republicans were an effort to bring about peace in Northern Ireland, 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 unified Ireland, The Daily Telegraph reported.
A spokesman for the Labour leader said the Security Service kept files on "many peace and labour movement campaigners" at the time.
Mr Corbyn's association with prominent republicans - including IRA terrorists - 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 that 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." He added: "I don't want violence, I don't want killing, I don't want all the horrors that go with it."