MP claims Downing Street link to abuse

Mr Watson, who has relentlessly pursued the Murdoch phone-hacking scandal, claimed a member of a notorious group was connected to a former Downing Street aide.

It was a bizarre new twist in the avalanche of allegations which have been made public in the wake of the Jimmy Savile affair at the BBC and elsewhere.

Mr Watson said: "The evidence file used to convict Peter Righton, if it still exists, contains clear intelligence of a widespread paedophile ring.

"One of its members boasts of his links to a senior aide of a former prime minister, who says he could smuggle indecent images of children from abroad.

"The leads were not followed up, but if the files still exists, I want to ensure that the Metropolitan Police secure the evidence, re-examine it and investigate clear intelligence suggesting a powerful paedophile network linked to Parliament and No 10."

Mr Cameron said it was a "very difficult and complex case" and he was "not entirely sure" to which former prime minister Mr Watson was referring.

He said he would look at the case "very carefully and see what the government can do to help give" Mr Watson the assurances he wanted.

Mr Righton, from Evesham, was a leading expert on residential childcare and was fined £900 in 1992 after admitting possessing and importing pornographic material including copies of a recognised paedophile magazine and photographs of naked boys under the age of 16.

There have been a number of high-profile cases over the last few decades that have come to the notice of the police.

In an article in 1998 Guardian reporter Nick Davies mentioned the late Peter Morrison, formerly MP for Chester and deputy chairman of the Conservative Party.

Mr Davies wrote: "Ten years ago, Chris House, the veteran crime reporter for the Sunday Mirror, twice received tip-offs from police officers who said that Morrison had been caught cottaging in public toilets with underaged boys and had been released with a caution."