The British government said today it would hold an inquiry into how a senior intelligence official left a file with top secret documents about Iraq and al Qaeda on a train.
A passenger found the orange folder, containing highly sensitive details about Iraq's security forces and the government's latest assessment of al Qaeda, on a London commuter service and handed it over to the BBC.
The two papers from the Joint Intelligence Committee had been with an unnamed official who worked in the Cabinet Office, the central government department that supports the work of prime minister Gordon Brown.
That official has now been suspended and Mr Brown said it was "a very serious incident".
"We will have to trace where these documents have gone, if they have gone anywhere other than in an envelope to a local BBC station," he told reporters.
Cabinet Secretary Ed Miliband told parliament the loss was a "clear breach" of security rules and an inquiry into the breach would be held, headed by former senior civil servant Sir David Omand, who dealt with security matters at the Home Office.
Mr Miliband said there was no evidence that Britain's national security had been compromised.
But the revelation is an embarrassment for Mr Brown, who has already been stung by accusations of lax security after a civil servant lost computer discs containing the names, addresses and bank details of 25 million people in the mail last year.