A secret file on “unnatural” sexual behaviour was not handed over to a Government backed review that looked for official documents linked to child abuse.

The file prepared for then Prime Minister Margaret Thatcher in the 1980s was found recently at the National Archives in Kew and is entitled: "PREM19/588 - SECURITY. Allegations against former public [word missing] of unnatural sexual proclivities; security aspects 1980 Oct 27 - 1981 Mar 20".

The Cabinet Office still holds the file. Thirty-five years after it was prepared it remains closed on grounds of national security.

The Chief Executive of the NSPCC, who led last year’s review into government documents from between 1979 and 1999, has said nobody showed it to him.

Peter Wanless told Sky News: "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.

"Our remit was to go back over a review by the Home Office and not undertake a new investigation."

Mr Wanless also joined a growing number of voices demanding that it should now be made available.

"If there is pertinent material in this file it should be submitted to the sexual abuse inquiry as well as the relevant police force so they can conduct a criminal investigation if necessary."

Security and intelligence expert Dr Chris Murphy stumbled across the file in November while searching documents at the National Archives in Kew and was immediately alerted by the title.

"I was looking through the 'PREM' Prime Minister file series for the 1980s," the University of Salford lecturer told Sky News.

"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."

It is highly likely Margaret Thatcher saw the documents, and was briefed on the security implications, but the identities of those within them remain secret.

Dr Murphy added: "The fact that this file is concerning the Prime Minister and these allegations are being taken to the Prime Minister I think would strike anybody of being potentially of some interest."

Sky News highlighted the existence of the file to the Cabinet Office earlier this month.

A spokesperson for the department said: "In this case, the file was kept closed and retained as it contained information from the security services and advice from the Law Officers.

"These classifications are reviewed periodically."

The department would not reveal whether David Cameron or Home Secretary Theresa May were aware of the contents, but promised it would be made accessible to the child sex abuse inquiry.

"We are clear that any files that are pertinent to the historical child sex abuse inquiry will be made available to the panel," the spokesperson added.

Mrs Thatcher's former press secretary, Sir Bernard Ingham, told Sky News he could not recall the file.

He did, though, confirm that both he and Mrs Thatcher were aware of allegations against a Government minister in the early 1980s.

Sir Bernard would not name the individual, but said: "I asked him about it and he denied it, so no, I didn't do anything else. What was the alternative?"

In July last year, the Prime Minister promised the survivors of sexual abuse that "no stone would be left unturned" in the pursuit of the truth.

Officials at the Cabinet Office have previously attempted to block requests for information relating to Cyril Smith , the late Liberal politician who is now known to have abused youngsters.

Simon Danczuk MP, who exposed the extent of Smith's offending, told Sky News the "sexual proclivities" file had to be released.

"I think it is right and proper that the Government now open up this document, let us know what is in there," he said.

"I think there is a culture within Government departments of not releasing information and that has caused some of the problems we now have in getting to the bottom of who was involved in this paedophile network and who was involved in covering it up."

The abuse inquiry was announced in July but currently has no chairman, after first Lady Butler-Sloss and then Fiona Woolf were appointed and then stepped down following concerns over their links to the political establishment.

It is still yet to appoint someone to lead it amid growing calls for action from survivors and campaign groups.