A coroner has refused to return secret documents mistakenly handed to him by the Ministry of Defence (MoD), a pre-inquest hearing was told.
14 servicemen died when their Nimrod crashed in Afghanistan
Oxfordshire assistant deputy coroner Andrew Walker got the material ahead of an inquest into how 14 servicemen died in a 2006 Afghanistan plane crash.
David Evans, acting for the MoD, told the hearing "highly-sensitive information had been released".
He said the papers should not have been sent out and should be returned unread.
Mr Evans said the matter was at the "highest level" and the information had been issued without permission.
MoD officials said they would visit the coroner's office to retrieve the material.
But Mr Walker said: "My concern is that if I surrender these documents, I am not going to see them again. They were given to me lawfully."
Mr Walker also refused a request not to read the material while the MoD prepared an application to have them revised.
The deaths of the servicemen were the heaviest loss of life to be suffered by British forces in a single incident since the Falklands War.
The inquest is not likely to start until 6 May after a further pre-inquest hearing.