A university lecturer has unearthed a previously top-secret file held at the National Archives which could contain allegations of ‘unnatural sexual’ behaviour by establishment figures in the 1980s.

Dr Chris Murphy, from Salford University, stumbled upon the potentially ‘extremely significant’ file by chance – weeks after a Home Office review into historic child abuse allegations failed to find any documents relevant to its investigation.

Dr Murphy, who is a lecturer in Intelligence Studies, was working on a research project about Government secrecy in the National Archives, in Kew, south West London, when he uncovered a file called: “PREM19/588 - SECURITY. Allegations against former public [word missing] of unnatural sexual proclivities; security aspects 1980 Oct 27 - 1981 Mar 20.”

The “PREM” category of files covers documents and correspondence that passed through the prime minister’s office.

He described how he did a ‘double-take’ when he saw the classified file – before wondering about the implications of the title.

Dr Murphy said: “This material was so significant that it was brought to the attention of Margaret Thatcher. How prominent must this individual have been in public life if it is being raised in Number 10?”

The academic raised the document with Rochdale MP Simon Danczuk, who has campaigned on behalf of victims of child sex abuse.

He said: “It’s astonishing that a Government review into child abuse allegations in the 1980s was unable to find any files that could help the inquiry and yet a Salford University lecturer has now managed to locate a file that could be extremely significant.

“All credit to Chris. Now the Cabinet Office need to publish the contents of this file and stop trying to sweep information relating to historic child abuse under the carpet.”

A Cabinet Office spokeswoman said classifications of filed are ‘reviewed periodically’.

She added: “In this case, the file was kept closed and retained as it contained information from the security services and advice from the Law Officers.” Asked whether it would be released to the current institutional child sex abuse inquiry, the spokeswoman said: “We are clear that any files that are pertinent to the historical child sex abuse inquiry will be made available to the panel.”

In November, a report was published following a review by NSPCC Chief Executive Peter Wanless into how the Home Office had dealt with allegations from Tory MP Geoffrey Dickens in the 1980s that prominent public figures had sexually abused children

It found ‘no record of specific allegations by Mr Dickens’.

That review ran along side the current institutional child sex abuse inquiry, which was set up by Home Secretary Theresa May in July to find out whether public bodies had neglected or covered up allegations of child sex abuse in the wake of claims paedophiles had operated in Westminster in the 1980s.

That inquiry has been beset by problems following the resignations of the Government’s first two choices for chairman and doubts over plans to give it extra powers. Previous appointments as inquiry chairwomen Fiona Woolf and Baroness Butler-Sloss resigned following claims about their perceived closeness to establishment figures.