MPs were stunned into silence when Mr Watson raised separate claims about a paedophile ring at the heart of the British establishment at Westminster – and demanded a police inquiry.
Mr Watson has previously led criticism of phone hacking in the media and helped uncover the extent of the practice among some newspapers.
His claims involve Peter Righton, who was convicted in 1992 by magistrates in Evesham, Worcestershire, of importing and possessing pornographic material involving boys under the age of 16.
Righton, who was 66 and lived in Badsey Road, Evesham, at the time of his conviction, had developed a reputation as an academic expert in child care, and had delivered lectures on the topic in Birmingham and elsewhere.
Mr Watson told the Commons: “The evidence file used to convict paedophile 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 file 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 Number 10.”
Righton, who belonged to the Paedophile Information Exchange, was convicted of importing child pornography from Holland in 1992