David Cameron has ordered the most senior official at the Home Office to conduct a fresh investigation into what happened to a missing dossier of alleged paedophile activity at Westminster in the 1980s.
The Prime Minister said he understood the concerns that had been raised about the missing file which was handed to the then home secretary Leon (now Lord) Brittan by the Conservative MP Geoffrey Dickens.
"That's why I've asked the permanent secretary at the Home Office (Mark Sedwill) to do everything he can to find answers to all of these questions and to make sure we can reassure people about these events," he said.
"So it's right that these investigations are made. We mustn't do anything, of course, that could prejudice or prevent proper action by the police.
"If anyone has information about criminal wrong-doing they should, of course, give it to the police."
Mr Sedwill has already commissioned one review into what happened to the dossier, after concerns were raised last year in Parliament and the media, which found it had not been retained "in line with departmental policy".
The review concluded the Home Office "acted appropriately, referring information received during this period to the relevant authorities."
Earlier, The Daily Telegraph reported that the dossier included the name of a former Tory MP who was found with child pornography videos but escaped without any action being taken.
The politician was allegedly stopped by Customs while driving back to the UK via Dover and found to have explicit videos of children "clearly under 12".
The Daily Telegraph reported that the politician was not arrested or charged following the incident in the 1980s and the videos and paperwork have subsequently gone missing
The Customs officer is said to have spoken to detectives on Operation Fernbridge, an investigation into allegations of sexual abuse by people including high-profile figures at the Elm Guest House in Barnes, south west London.
Labour MP Tom Watson, who has led calls for a comprehensive inquiry into historic child abuse, said he was writing to DPP Alison Saunders to ask her to examine the evidence relating to the unnamed Tory politician.
He said: "It's a remarkable revelation. If true, it shows that a crime was not investigated but also is shocking because it's yet another example of intelligence going missing.
"I hope the DPP will share the concern of many MPs about this.
"It has got to be fully investigated and also shows why there are a growing number of calls from people for a wider overarching inquiry into historic allegations of abuse."
The Metropolitan Police said: "We are not prepared to give a running commentary on Operation Fernbridge, which is an ongoing operation."
Meanwhile, it emerged that four more cases of historic sex abuse have been referred to the police by Home Office officials in recent months, following a review ordered last year covering the period 1979 and 1999.
A review of a database containing details of more than 746,000 files identified 13 items of information about alleged child abuse, including four cases involving Home Office staff.
Nine of these items of information, including all of the cases involving Home Office officials, were either already known to the police or were reported to them by the Home Office at the time.
The remaining four have now been passed to the police for a "proper assessment", although the investigator who carried out the review said the information was likely to be of "limited value".
A Home Office spokesman said: "In response to concerns raised in Parliament and the media relating to the handling by the department of historical allegations of abuse, the permanent secretary commissioned an independent review of all relevant papers received by the department between 1979 to 1999 to identify any information received and the outcome.
"The review concluded the Home Office acted appropriately, referring information received during this period to the relevant authorities."
Home Affairs Select Committee chairman Keith Vaz has asked the department for further information and the Home Office said: "It would be inappropriate to comment further until we have responded to the chair's request."
Simon Danczuk, MP for Rochdale, said: "The Prime Minister knows that there is a growing sense of public anger about allegations of historic abuse involving senior politicians and his statement today represents little more than a damage limitation exercise. It doesn't go far enough.
"The public has lost confidence in these kind of official reviews, which usually result in a whitewash. The only way to get to the bottom of this is a thorough public inquiry."