The British government's problems with missing files deepened dramatically when the Foreign Office claimed documents on the UK's role in the CIA's global abduction operation had been destroyed accidentally when they became soaked with water.

In a statement that human rights groups said "smacked of a cover-up", the department maintained that records of post-9/11 flights in and out of Diego Garcia, the British territory in the Indian Ocean, were "incomplete due to water damage".

The claim comes amid media reports in the US that a Senate report due to be published later this year identifies Diego Garcia as a location where the CIA established a secret prison as part of its extraordinary renditionprogramme. According to one report, classified CIA documents state that the prison was established with the "full cooperation" of the UK government.

It also comes at a time when MPs are demanding the Home Office urgently provide more information about 114 "missing" files that could have contained information about an alleged child abuse network in the 1980s.

Ministers of successive governments have repeatedly given misleading or incomplete information about the CIA's use of Diego Garcia. In February 2008, the then foreign secretary, David Miliband, was forced to apologise to MPs and explain that Tony Blair's "earlier explicit assurances that Diego Garcia had not been used for rendition flights" had not been correct. Miliband said at this point that two rendition flights had landed, but that the detainees on board had not disembarked.

Miliband's admission was made after human rights groups produced irrefutable evidence that aircraft linked to the rendition programme had landed on Diego Garcia. Since then, far more aircraft have been shown to have been involved in the operation.