Scotland Yard has been called in after Home Secretary Theresa May's personal engagements diary was mislaid.
Britain's biggest force is looking into the circumstances of how the diary went missing, but insisted that security was not compromised.
A force spokesman said: "We are aware that a document was misplaced and are looking into the circumstances of how this occurred. The paper was not protectively marked. Security was not compromised."
Keith Vaz, Labour chairman of the Commons Home Affairs Select Committee, said: "It's serious that someone with the security importance of the Home Secretary should have her security compromised in this way. It is very disappointing that this has happened."
It is understood the diary was left at a Glasgow concert hall on Sunday where Mrs May was attending the National Police Memorial Day to remember fallen officers, which was also attended by the Prince of Wales.
It emerged later that the five-page document was found by Royston Martis, deputy editor of the Police Review magazine, at 5.30pm on Sunday, after the Home Secretary and her security team had left the event at Glasgow Concert Halls.
It has been returned to the Home Office and no details of specific events were published until after they had taken place.
The document said at what time and on which days Mrs May would be going to the gym in her Maidenhead, Berkshire, constituency, the magazine said.
Detailed timings and addresses for events Mrs May was to attend in the Maidenhead area, including the opening of Chissock Woodcraft, near Reading, and a charity cabaret evening at Wentworth Golf Club, were also given, along with mobile telephone numbers for Mrs May's private secretary and key contacts at the events.
The words "News of the World", the newspaper axed in the wake of the phone hacking scandal, were also scribbled in black pen on its front page.