Margaret Thatcher was made aware of child abuse allegations involving Cyril Smith before he was knighted in 1988, Cabinet Office documents have shown.
The papers, first released to the Mail on Sunday, also show the then PM was warned the award could risk "the integrity of the honours system".
The newspaper has complained over the time it took to release the documents, which followed five requests.
But the Cabinet Office says there had been "no cover up" of the information.
The Mail first requested the documents under the Freedom of Information Act in April last year but they were only released on Friday, which the paper says came after it made a complaint to the Information Commissioner.
The dossier on the decision to confer a knighthood on Smith, a former Rochdale MP for both for the Liberal Party and the Liberal Democrats who died in 2010, runs to 19 pages.
It includes one undated letter marked secret, from Lord Shackleton, a member of the Political Honours Scrutiny Committee.
This was sent to Mrs Thatcher - who died in 2013 - and included a warning of "the risk that such an award could give rise to adverse criticism".
Lord Shackleton wrote that police had investigated Smith in 1970 for "indecent assault against teenage boys" between 1961 and 1966, but the director of public prosecutions (DPP) had decided "there was no reasonable prospect of conviction".
He went on to say that the case was reported in the Rochdale Alternative Press and Private Eye.
"One may regret this kind of press reporting but it could be revived if an award to Mr Smith were made," he added.
Lord Shackleton said it would be "slightly unfortunate" if this "episode" stopped Smith receiving the honour.
However, he added: "We felt it right to warn the honours system would be at some risk if the award were to be made and announced."
A second note to the prime minister, dated May 1988, said the committee had "some hesitation" about awarding the knighthood but "so far as we believe and have been able to ascertain, his past history or general character does not, in all the circumstances, render him unsuitable".
And the committee's secretary said, in another letter, that Smith had been given the "benefit of the doubt" because he had not been prosecuted.
Another document in the dossier, from then cabinet secretary Sir Robin Butler - now Lord Butler of Brockwell - shows he wrote to the DPP on the committee's behalf, requesting more information about Smith.
He said: "The case for taking the exceptional step of writing to you in this way is to protect the Prime Minister (and The Queen) while also being fair to Mr Smith."
The committee, he wrote, wanted to know "whether the case against Mr Smith was not well founded: or whether it was a sound case, but that the evidence was not likely to stand up in court".
According to the Mail, the file does not record any reply from the DPP.
A Cabinet Office spokesman said: "There is no cover up nor was the Cabinet Office forced to release this information by the Information Commissioner.
"This is a sensitive and complex case and it is right that we considered advice from a range of officials. After considering the advice, the Cabinet Office decided to disclose information."