A police investigation has been launched after secret Government documents were left on a train, it was confirmed today.
It is understood that the two documents - both marked "Secret" - relate to al-Qa'ida in Pakistan and the security situation in Iraq and were lost yesterday.
The documents were eventually handed to the BBC's security correspondent Frank Gardner, who reported details of the security breach.
A spokesman for the Cabinet Office said: "Two documents which are marked as secret were left on a train and have subsequently been handed to the BBC. There has been a security breach, the Metropolitan Police are carrying out an investigation."
The reports are assessments made by the Government's Joint Intelligence Committee, said the BBC.
The Cabinet Office spokesman said the papers had been in the possession of a senior intelligence officer based in the Cabinet Office.
Asked how many people would have had access to the papers, he said: "'Secret' is a high classification so they would have had limited circulation."
The spokesman declined to discuss the contents of the documents.
Gardner said the documents were left in an orange cardboard envelope on a train from London Waterloo to Surrey by a "very senior intelligence official" working in the Cabinet Office.
A full-scale police search was launched when it was realised that they were missing, as officials were concerned at the possibility of such sensitive papers finding their way into the wrong hands, he said.
The incident is the latest in a series of embarrassing losses of Government information, including the disappearance of personal details of millions of child benefit recipients on a disc sent through the post.
The envelope was in fact picked up by a fellow passenger, who looked inside and found a seven-page document setting out the latest Government assessment on the Islamist terror network al-Qa'ida, along with a "top secret and in some cases damning" assessment of Iraq's security forces, said Gardner.
The al-Qa'ida document, commissioned jointly by the Foreign Office and Home Office, was classified "UK top secret", he said. It was so sensitive that each page was numbered and marked "For UK, US, Canadian and Australian eyes only".
The second document, on Iraq, was commissioned by the Ministry of Defence.
Gardner said: "This was a clear breach of Government rules. They should be sealed in a briefcase if they are taken out."
A Metropolitan Police spokesman said: "We are making inquiries in connection with the loss of documents on 10 June."