Liberal MP Cyril Smith wrote to the BBC in 1976 asking it not to investigate the "private lives of certain MPs".

The MP, who died in 2010 and has been accused of abusing children, wrote to the then home secretary about "filth, innuendo and stirring" by reporters.

The BBC investigation had been looking into claims of an alleged foreign-backed campaign to discredit MPs.

Former children's minister Tim Loughton said the former Rochdale MP's letters were "bully-boy tactics".

"It was an abuse of position that somebody as an MP was saying, 'You shouldn't look at us, we're above the law,'" he said.

'Personal involvement'

Smith had been the subject of an investigation into the alleged abuse of children in Rochdale but the case was not known about publicly, and he was never charged.

Current Rochdale MP Simon Danczuk is due to appear before the Home Affairs Select Committee later, where he is expected to call for a new inquiry to include the activities of Smith.

Mr Danczuk recently published a book alleging more than 140 complaints had been made by victims but Smith had been left free to abuse children as young as eight.

Greater Manchester Police and Rochdale Council are carrying out two separate investigations into child abuse allegations involving the late MP.

More than 100 MPs are calling for a larger inquiry into historical claims of child abuse in schools, hospitals and care homes.

At the time the media had been investigating a claim made by Prime Minister Harold Wilson that South African secret agents had been trying to smear British MPs.

The Liberal Party was thought to be a particular target because of its outspoken opposition to South Africa's apartheid policy.

The BBC had employed two freelance journalists, Barrie Penrose and Roger Courtiour, to look into Mr Wilson's claims.

According to letters in the National Archives, Smith wrote to BBC director general Sir Charles Curran in September 1976 saying he was "deeply concerned about the investigative activities of the BBC", especially relating to "the private lives of certain MPs".

"So far as I am aware I am not one of them, and hence I write without personal involvement."

Public interest

In another letter, Smith urged the then Home Secretary, Merlyn Rees, to ensure the BBC was not using public money for "muck-raking".

He wrote: "Frankly, I am fed up of the filth, innuendo and stirring that has gone on for the last six months or so about MPs in all three political parties, and I really do think the time has come for something to be published, or for the thing to come to an end."

The month before Smith wrote his letter, the BBC had ended its contract with Penrose and Courtiour, saying it did not believe it had proper control over where else they might publish their material.

It is understood Smith had not been one of the subjects of their investigation.

Sir Charles responded to Smith, saying the South African story was "a proper subject for journalistic inquiry".

But he added: "I was not prepared to see public resources devoted to the pursuit of personal dirt, possibly for publication outside the control of the BBC."

But Roger Courtiour said the two journalists were "totally convinced the story was in the public interest and should have been continued".

Image caption Part of the BBC director general's memo relating to the South African investigation

On Tuesday, the BBC said: "The documents date back nearly 40 years, so we have no additional commentary to offer, and their content appears to be self-explanatory."

The Home Office said it would make any decisions about a further inquiry into child abuse after a number of current investigations were complete.