British police have opened an investigation into how top-secret anti-terrorist security plans for London's Heathrow airport apparently ended up on roadside.

London's Metropolitan Police are trying to find out how and why the confidential security document was left where anyone could find it.

The probe opened after the Sun newspaper told police it had received the security plan from a businessman, who said he found it on a roadside near a service station, a few hundred meters from Heathrow airport.

Home Secretary David Blunkett told British radio the documents appear to be genuine, and he is awaiting more information from Metropolitan Police Commissioner John Stevens.

"Well, the plans were obviously very good," said Mr. Blunkett. "Somebody disposing of them in a way that allowed that to happen is very bad, and the commissioner will be reporting on the investigation into how that happened."

According to the Sun's report, the document written last month by the police anti-terrorist Aviation Security team contained detailed maps and photographs of 62 potential terrorist missile launch sites near Heathrow.

The newspaper says the document also contains confidential information on various anti-terrorist measures, including the times of police patrols, the deployment of rooftop snipers and the use of dog units.

It says other details include possible escape routes of terrorist attackers and police plans for evacuation routes and road closures.

Heathrow is one of the world's busiest airports, handling about 65 million passengers a year.

Terrorists have hit Heathrow in the past, and police believe it remains a high-priority target. Irish republican attackers fired three mortar rounds at the airport during a five-day period in 1994, but none of the shells exploded. A Pan American airlines flight from Heathrow to New York blew up over Scotland in 1988, killing 270 people.