Top-secret documents containing the latest government intelligence assessment on al-Qaeda have been left on a train in London.

The documents belonged to a very senior intelligence official working in the Cabinet Office.

A passenger on the train from Waterloo to Surrey spotted the orange cardboard envelope lying abandoned on a seat and handed the documents to the BBC.

A full-scale search had been launched by the Metropolitan Police.

Just seven pages long but classified as "UK Top Secret", the latest government intelligence assessment on al-Qaeda is so sensitive that every document is numbered and marked "for UK/US/Canadian and Australian eyes only", BBC security correspondent Frank Gardner said.

However, it appears that in a "serious" breach of the rules, according to our correspondent, the document was taken out of Whitehall and left on a train on Tuesday.

'Damning assessment'

When a fellow passenger saw the material inside, which included a top secret and in some places "damning" assessment of Iraq's security forces, they handed it in to the BBC.

Meanwhile a full-scale search had been launched by the Metropolitan Police, amid fears that such highly sensitive material could have fallen into the wrong hands.

The two reports were assessments made by the government's Joint Intelligence Committee.

The report on Iraq was commissioned by the Ministry of Defence and the one on al-Qaeda was commissioned jointly by the Foreign Office and the Home Office.

Our correspondent said that across several departments in Whitehall this evening there is said to be "horror" that top-secret documents could have been so casually mislaid.