Home Secretary David Blunkett, Britain's top law enforcement official, said he believed the documents were genuine.
"The plans were obviously very good," he told British Broadcasting Corp. radio. "Somebody disposing of them in a way that allowed that to happen is very bad."
London's Metropolitan Police refused to give details of the nature of the documents, but said they had been recovered by officers.
"We treat any breach of security extremely seriously," a spokesman said, speaking on condition of anonymity.
"We've launched an internal inquiry into the circumstances of how these documents went missing, and we will take the appropriate action when we have ascertained the facts surrounding this matter."
The spokesman was responding to a report in The Sun daily claiming that a motorist found the documents in a road.
The newspaper said the plans, dated June 26, 2004, showed 62 sites at Heathrow where al Qaeda terrorists were most likely to launch anti-aircraft missile strikes. They included facts about escape routes, evacuation plans and road closures, according to The Sun.
Published under the headline "Dossier of Death," the Sun's story also revealed details of Heathrow security operations, like the use of dog patrols and rooftop snipers.
British aviation security has been high since the Sept. 11, 2001, terror attacks. In February 2003, British troops and tanks were deployed at Heathrow Airport after police warned that al Qaeda might try attacking the British capital.
Despite the increased security, Heathrow has been the site of security breaches since the Sept. 11 attacks. In February 2002, robbers stole several boxes of foreign currency from a van in a high-security area of the airport.
According to Airports Council International, Heathrow is the world's third-busiest airport in terms of number of passengers served annually, with 63.4 million in 2003. Atlanta's Hartsfield-Jackson airport and Chicago-O'Hare serviced more people.