A classified government file describing “unnatural” sexual activity occurring in Westminster circles in the early 1980s may have been seen by ex-Prime Minister Margaret Thatcher, an intelligence expert says.

The documents were stored in the ex-PM’s files, suggesting she may have seen them during her premiership.

A spokesman for the Cabinet Office told RT on Wednesday the dossier will be sent to Britain’s inquiry into historic child sex abuse.

The sensitive document was discovered by security and intelligence expert, Dr Chris Murphy, who was suspicious of its title: "PREM19/588 -SECURITY. Allegations against former public [missing word] of unnatural sexual proclivities; security aspects 1980 Oct 27 - 1981 Mar 20."

Murphy told Sky News he found the file when he was sifting through the “'PREM' Prime Minister file series for the 1980s.”

“I think I did a double-take and then started wondering what the potential implications of the title, which is a little vague, could be,” he said.

The precise details of the file have not been publicly disclosed. A Cabinet Office spokesman said the document won’t be released to the public on the grounds of national security.

"It is not unusual for the National Archives to identify in its catalogue records either closed or retained by government departments," he told RT.

"In this case the file was kept closed and retained as it contained information from the security services and advice from the Law Officers.”

Thatcher’s former press secretary, Sir Bernard Ingham, told Sky News he has no recollection of the file.

But he admitted he and Thatcher were aware of allegations made against a minister in the early 1980s.

Reflecting on the matter, Ingham said he approached the minister in question at the time, and he categorically denied the allegations.

Ingham said he took no further action, suggesting there was no alternative.

The secret file documenting strange sexual behavior in Westminster was never given to a Government-backed review that specifically looked for state documents potentially linked to child abuse.

The NSPCC’s Chief Executive, Peter Wanless, who headed last year's review into government documents dated 1979-1999, has confirmed no one showed it to him.

"Under the specific terms of reference set by the Home Office we made the most extensive inquiries possible within a very limited time frame. This specific file was not revealed by any department or individual we consulted,” Wanless told Sky news.

The file remains in the possession of the Cabinet Office.

Britian's inquiry into historic child abuse has been the subject of intense criticism of late, and is accused of lack of robustness.

In late December, Labour MP John Mann urged the home secretary to address state secrecy surrounding historic child abuse in Britain.

Mann believes the Official Secrets Act is obstructing ex-police officers from stepping forward with vital information relating to allegations regarding a child sex ring affiliated with powerful Westminster elites throughout the 1970s and 1980s.