Downing Street has denied that security was breached after David Cameron left his official ministerial box on a train table while he was travelling to a wedding.
A fellow passenger took a photograph of the well-worn red box, with its key in the lock, while the Prime Minister was in another part of the train.
"It was just sitting there. I could probably have run off with it if I'd wanted to," they told the Daily Mirror.
But Downing Street insisted 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.
The boxes are used to transport official documents, speeches and letters to be signed. Their design has changed little since the 1860s.
More discreet black boxes are said to be available for ministers travelling by train.
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."
The snap was taken on Saturday as Mr Cameron - fresh back from the G20 summit in Russia - took the train from King's Cross, London, to York to attend his sister-in-law's wedding.
Last year, the Prime Minister left his daughter behind after visiting a pub near Chequers after confusion over which car she was travelling in.
She was reunited with the family when Samantha Cameron went back after 15 minutes.