More than 100 official files relating to historic allegations of organised child abuse have gone missing, the Home Office has disclosed.
Mark Sedwill, the permanent secretary at the Home Office, said the documents - which related to a 20-year period between 1979 and 1999 - were "presumed destroyed, missing or not found".
The disclosure came as Mr Sedwill said that he was appointing a senior legal figure to carry out a fresh review of how the Home Office dealt with a dossier alleging paedophile activity at Westminster in the 1980s.
He was responding to a call from David Cameron who urged him to "do everything he can" to establish what happened to the file which was handed to the then Home Secretary Leon (now Lord) Brittan by Tory MP Geoffrey Dickens in 1983.
Only last year, a review - carried out for the Home Office by a HM Customs and Revenue investigator - concluded the relevant information in the file had been passed to the police and the rest of the material destroyed in line with departmental policy at the time.
In a letter to the Prime Minister, Mr Sedwill wrote: "To provide the additional reassurance you are seeking, particularly in view of information now in the public domain since the investigation's conclusion last year, I will engage a senior independent legal figure to assess whether the review's conclusions remain sound. I aim to make the appointment within the next week."
In a separate letter to the chairman of the Commons Home Affairs Committee, Keith Vaz, Mr Sedwill disclosed that while the original review had identified 527 potentially relevant files which had been retained, there were a further 114 files which could not be located.
He said that the investigation had not found a single dossier from Mr Dickens, but several sets of correspondence over a number of years to a number of home secretaries containing allegations of sexual offences.
However he said that the review had found no record of specific allegations by Mr Dickens of child sex abuse by prominent public figures.
"Like any other citizen, I am horrified by what we have learnt in the past couple of years about the systematic abuse of children and vulnerable adults by prominent public figures, and the state's failure to protect them," he wrote.
"Some have been brought to justice and I hope that the police investigations now under way across the country are equally successful. The Home Office has and will co-operate fully with any police inquiry."
The disclosures have intensified calls from MPs for Mr Cameron to hold an over-arching inquiry into all the allegations of historic child sex abuse from that period.
The pressure has been growing since Labour MP Simon Danczuck - who has carried out his own investigation into child abuse by the former Liberal MP Sir Cyril Smith - raised the issue of the Dickens dossier at a Home Affairs Committee hearing.
Shadow home secretary Yvette Cooper welcomed the latest review but said that it still did not go far enough.
"The Home Secretary doesn't seem to have grasped the gravity of this and so officials and Downing Street have not yet taken the action we need," she said.
"Given the many different inquiries, Theresa May must establish an overarching review led by child protection experts. This would draw together the results from all the different cases, investigations and institutional inquiries to allow us to learn from the failure of previous decades and keep children safe in the future."