It is a fact that conspiracy theorists have always dreamed about. Top secret files on Princess Diana and her closest associates are held by the United States national security agency - and that is official.

The agency has told the Guardian that it is holding reports from foreign intelligence - thought to include MI5 and MI6 - under both top secret and secret categories. It revealed their existence after the Guardian filed a request under the US freedom of information act. The reports cannot be released because of "exceptionally grave damage to the national security".

The documents on the dead princess seem to have arisen because of the company she kept rather than through any attempt to target her, and the agency goes out of its way to say that it did not compile any of the spy reports itself.

"The reports contain only references to Princess Diana acquired incidentally from intelligence gathering. It is neither NSA policy or practice to target British subjects in conducting our foreign intelligence mission. However, other countries could communicate about these subjects; therefore, this agency could acquire intelligence concerning British subjects," an agency statement said.

The agency rejected the Guardian's request to release the files on two grounds. As well as warning that "the documents are classified because their disclosure could reasonably be expected to cause exceptionally grave damage to the national security," the agency also says it needs to protect its sources.

The existence of documents on Diana, Princess of Wales has always been claimed by the owner of Harrods, Mohamed Al Fayed, whose son, Dodi, died with the princess in a car crash in Paris in 1997. He believes reports which claim that up to 1,056 page references to Diana exist in some 39 documents held by the NSA.

Mr Fayed has started a welter of court actions in Washington and Baltimore in an attempt to force the NSA, the defence intelligence agency, and the CIA to release documents which he believes could throw light on the death of the princess and his son.

He is also suing a group which demanded money for fabricated documents claiming that the intelligence services were involved in a plot to assassinate the princess and her boyfriend that night. It is not known whether these documents were held by the NSA, nor whether claims published last year that the agency was snooping on her love life were correct.

Under Jack Straw's freedom of information bill Britain's security services would not be able to confirm or deny that they were holding documents on Princess Diana. The services are covered by a blanket ban exempting them from disclosing even documents' existence.