A commuter found the dossier and handed it to The Sun newspaper, the red-top reports.
A chief inspector in Scotland Yard’s Territorial Policing branch is apparently “hugely embarrassed” by what could have been a very costly mistake.
The documents set out security plans for the sites of events and, perhaps even more damaging, notes from top-level meetings on how to beat terrorists.
The Sun reports that had the dossier fallen into the wrong hands, it would have enabled terrorists to “mount a devastating attack on the Games in London”.
The plans even give detailed explanations of emergency “lock-down” procedures.
In a further threat to security, the dossier reports on complaints from officers about the radios they will use during the Olympics.
One cop says: “Radios NEVER work properly.”
The Sun quoted the commuter who found the documents as saying: “I couldn’t believe any policeman could have left this on a train. It’s a worry.”
A spokeswoman told the paper: “An officer lost his bag containing a number of documents. He reported the loss.
“The Directorate of Professional Standards have been informed, as is routine.”