Sensitive documents setting out Theresa May's travel plans were left on a train in a blunder branded a "serious security breach".
Downing Street has launched an investigation after the details of the Prime Minister's schedule were found by a passenger.
As well as setting out transport arrangements for a visit to the North West, the papers included the address of a hotel Mrs May was booked in to work from during the afternoon.
They also outlined plans for a call to Nato Secretary General Jens Stoltenberg as well as arrangements for a "treasurers' dinner" in Altrincham.
The documents, discovered in the first class train carriage of a train heading for Edinburgh on the day of the visit, were handed to The Daily Mirror.
Former royal bodyguard Inspector Ken Wharfe told the newspaper that leaving travel documents on a train was "inexcusable" and a "serious security breach".
He said: "In today's environment, it becomes a serious security matter when documents like this detailing the hourly movements of a protected person are misplaced. That's just not acceptable.
"Theresa May is given this protection because she is a target and so, of course, this is a serious security risk.
"These papers give a detailed -assessment of her movements. Had such information reached potential terrorists it could serve to confirm their plans.
"But also, importantly, it would highlight the lapse and suggest to those with such terrorist intentions the weakness surrounding their potential target."
Mrs May had flown from RAF Northolt, West London, to Manchester before heading to a science park in Warrington for the January 23 visit.
After meetings and questions from the media, the plan was to travel by car to The Mere Hotel in Knutsford and later fly back to London after dinner.
A Government spokesman said: "We have been made aware of claims around a Government document and will investigate accordingly."