The home secretary has said the disappearance of papers reported to contain counter-terrorism plans for Heathrow Airport was "very bad".
Heathrow is a large potential target for terrorist attacks
A report in the Sun newspaper claimed the documents identified 62 sites from which a missile strike could be made.
They were found by a motorist in a layby near the London airport who contacted its reporters, the Sun said.
The plans have been returned to the Metropolitan Police which has launched an internal inquiry.
The files were dated 26 June 2004 and included details about escape routes, evacuation plans and road closures, according to the newspaper's Monday edition.
Confidential details of police measures such as patrol
times, use of dog units and deployment of rooftop snipers were also set out, it said.
Home Secretary David Blunkett told the BBC Radio 4 Today programme: "As far as I know this is a genuine document."
He added: "The plans were obviously very good. Somebody disposing of them in a way that allowed that to happen is very bad.
"The Commissioner [of the Metropolitan Police, Sir John Stevens] will be reporting on the investigation as to how that happened."
A Scotland Yard spokeswoman could not confirm reports the dossier was compiled by the SO18 Aviation Security team, which is based at Heathrow police station.
She said: "We treat any breach of security extremely seriously.
"We have launched an internal inquiry into the circumstances of how these documents went missing and will take the appropriate action when we have
ascertained the facts surrounding the matter."
She said she could not discuss the contents of the papers nor where they were found "for security reasons".
John Stewart, chairman of Hacan Clear Skies, a pressure group for people living under the Heathrow flight path, said there was concern in the local community over a possible attack.
"Heathrow is in the most built-up area of any airport in Europe," he said.
"If there's a terrorist attack that was successful, it wouldn't only take out planes, it would take out hundreds or thousands of homes. Maybe tens of thousands of people would be made homeless and worse."