Gordon Brown has promised a full investigation after top-secret Whitehall security documents were left on a commuter train.
The Prime Minister admitted the incident was "very serious" and said the Government was trying to establish how widely the files relating to al Qaida and Iraq had been circulated.
Speaking at his regular Downing Street press conference, Mr Brown insisted: "Of course we take this seriously and of course any breach of intelligence cannot be condoned. We will inquire and report on all the circumstances in which this happened."
He added: "We will have to trace where these documents have gone, if they have gone anywhere apart from in an envelope to a local BBC station."
The official at the centre of the breach has been suspended.
Meanwhile, Cabinet Office Minister Ed Miliband has told the House of Commons the loss of documents was a "clear breach" of security rules.
Mr Miliband told MPs the official - a member of the Joint Intelligence Committee assessment staff - had no authorisation to remove the files from Government premises.
Former Permanent Secretary for Security and Intelligence Sir David Omand will carry out a full investigation of the circumstances of the case, he added.
However there was no evidence to suggest vital national security interests had been damaged, or that any individuals or operations were at risk.
Tory spokesman Francis Maude said: "There can scarcely have been a graver breach of intelligence and security procedures than this."
