Documents listing Prime Minister Theresa May's travel plans were left behind on a train in a serious security breach, it has been revealed.

Downing Street launched an investigation after a passenger found the documents which outlined travel arrangements for the PM, the Telegraph reports.

In addition to detailing a visit to the North-west, the documents listed the address of the hotel that May was booked into to work from during that afternoon.

They also gave details of plans for a call to Nato Secretary-General Jens Stoltenberg and arrangements for a party treasurers dinner in Altrincham, Cheshire.

According to the newspaper, the documents were found in the first class carriage of a train heading towards Edinburgh. They were handed to the Daily Mirror.

A government spokesman said: "We have been made aware of claims around a government document and will investigate accordingly."

May flew from RAF Northolt in West London to Manchester before visiting a science park in Warrington for the 23 January visit.

She was then to travel by car to the Mere Hotel in Knutsford and fly back to London after dinner.

Conservative Party's Scottish conference
Theresa May: Security breached Russell Cheyne/Reuters

A former royal bodyguard, Insp Ken Wharfe, told the Telegraph: "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.

"Had such information reached potential terrorists it could serve to confirm their plans."