Tom Watson, MP for West Bromwich East, was the first to report claims of an establishment paedophile ring.
And now it has been revealed MI5 had known about the allegations but failed to investigate after an MP during the Thatcher era had been accused of being a paedophile.
Sir Antony Duff, then MI5 director-general, wrote to Lord (then Sir Robert) Armstrong, the cabinet secretary, saying the man 'has a penchant for small boys'.
He added: "At the present stage the risks of political embarrassment to the government is rather greater than the security danger."
The letter was among files discovered by the Cabinet Office and handed to the official inquiry into historical abuse allegations.
But campaigners believe it shows a deliberate cover-up.
The MP accused of abuse has not been named.
Labour deputy leadership candidate Mr Watson said: "For the first time we have a document that shows Britain that top civil servants and the intelligence service was aware of allegations that MPs were involved in child abuse and seemed to have shown more concern for the reputation of the government than ensuring justice was done for their victims.
"We are getting closer to the truth but we are not there yet."
Meanwhile investigations are continuing into allegations made by 32-year-old Esther Baker that she was abused on Cannock Chase as a child in the 1980s and 1990s.
She has waived her right to anonymity after claiming an MP and a peer were among a group of people who routinely abused her at Cannock Chase while uniformed police stood guard.
The former MP concerned has not been named but has utterly denied the allegations.
Miss Baker told the Express & Star she believed that the authorities were taking allegations against politicians seriously.
She said: "People are now not taken by surprise when these allegations come to light. I think that is to do with cases such as Jimmy Savile. Once they would have been shocked and would not have believed it.
"Now people have come to terms with this and those who come forward are more likely to be taken serious. That is welcome."
David Bailey, spokesman for Staffordshire Police, said: "There is no change and the investigation on going.
"We don't have any further update at this stage and our previous statement stands."
The force previously said it had 'received a small number of calls following the coverage which we will be following up'.
It said it was unable to confirm the nature of these calls 'at this early stage of the investigation'.