Up to 10 million pages of vital military secrets have been rendered unusable by exposure to asbestos - and experts say the contamination threatens the operation of the Freedom of Information Act.

The 63,000 files include many nuclear secrets and the official versions of events such as the sinking of the Belgrano in 1982 and the killing of IRA terrorists in Gibraltar by the SAS in 1988.

A decontamination expert said the cost of cleaning the files would run into tens of millions of pounds and could take years to complete.

The Ministry of Defence says the files have been removed from the basement of the old War Office building in Whitehall to a warehouse in west London while it works out how to deal with them.

It has known about the contamination at least since spring last year but the first meeting of a committee to decide how to make the documents safe for people to read was held only last month. The delay has alarmed watchdog groups. They fear that many files will be excluded from public scrutiny for indefinite periods.

Under the Freedom of Information Act, which comes into force on Jan 1, all such files can be subject to requests by members of the public. Campaigners for open government fear that the ministry will now be able to delay answering such requests.

Maurice Frankel, the chairman of the Campaign for Freedom of Information, said:"This is a very serious problem and the Ministry of Defence must give it priority, given that the Freedom of Information Act is coming into full force imminently."

Historians who have asked to see some of the files under less sweeping open government codes have been told it is impossible and that the ministry cannot say when"normal" procedures will be resumed. Under the Freedom of Information Act, the ministry is allowed only 20 working days before having to say whether a researcher can see particular files.

Prof Matthew Jones, of Nottingham University, an expert on the Cold War and Britain's nuclear history, said:"I find it disturbing that the MoD may be able to use the excuse of asbestos not to fulfil the requirements of the Act.

"These files are irreplaceable records of this nation's defence and foreign policy during the 20th century."

A ministry spokesman said:"The immediate concern of those responsible for the records has been to protect the health and safety of staff and others.

"Therefore the records have been treated as potentially contaminated with asbestos and sealed into plastic sacks which were then packed into crates."

Bill Sanderson is a decontamination expert with experience of cleaning hundreds of files belonging to the London Docklands Development Corporation that were exposed to asbestos in 1993.

He said:"This is a very substantial undertaking. Unless documents are thought to be very valuable, we would recommend destroying them. That does not seem to be possible here."

Asked how much it would cost and how long it would take to decontaminate all the files, he said he could only estimate"many millions of pounds and possibly years".

Prof Peter Hennessey, of Queen Mary's College, London, an expert in Whitehall secrecy, said:"Now that we know about this, they cannot destroy the files under the Public Records Act. But I gather that consideration was given to destruction in the first instance.

"These are the crown jewels of the Cold War generation and we owe it to that generation of people who did many serious and secret things for their country to get this stuff out."

The ministry said that documents would not be destroyed unless they had been transferred to"another medium" - scanned into computers or microfilmed - for permanent preservation at the National Archives.

A source at the National Archives said:"People are understandably suspicious because the Government generally and the MoD in particular do not have a very good record in handling secrets."