Downing Street has denied reports that the Prime Minister breached security by leaving his ministerial box unattended on a train table in order to visit the buffet car.

A fellow passenger on the train took a photograph of the well-worn red box on a train table with its key still in the lock.

"It was just sitting there. I could probably have run off with it if I'd wanted to," the passenger told the Daily Mirror.

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The newspaper published a picture snapped by the passenger of the beaten-up looking box on the train table.

Downing Street has insisted, however, that the box was "not left unattended" as the premier's security detail "was there at all times".

In 1999 Labour minister Peter Kilfoyle issued guidance that red boxes should be "effectively disguised" if it was necessary to carry them on public transport.

Labour MP James Purnell was forced to apologise after he sparked a security alert after losing confidential documents on a train.

Transport Minister Simon Burns also prompted debate by suggesting he could not use public trains because he wasn't allowed to work on confidential papers in the carriages.

Mr Burns found himself at the centre of a row over his use of an £80,000-a-year chauffeur-driven government car to commute daily between his Essex home and Westminster.

Chancellor George Osborne famously had train trouble of a different kind last year after he told an inspector he 'could not possibly' sit in standard class but could not pay any extra.

The incident was famously dubbed the 'great train snobbery'.

Former deputy prime minister John Prescott told the newspaper: "I'm staggered that a prime minister should be so slack about looking after Government secrets.

"The box could have contained detailed confidential intelligence about Syria. I never let mine out of my sight. Talk about Inter Twitty. The guy needs to get a grip."

The snap was taken on Saturday as Mr Cameron - fresh from the fraught G20 summit in Russia - took the train from King's Cross, London, to York to attend his sister-in-law's wedding.

Mr Cameron also faced press scrutiny last year following reports he left his eight-year-old daughter Nancy at the Plough Inn in Cadsden 'for about 15 minutes' after a mix up.