"The immediate threats are not verifiable, therefore we are working on finding out more information," Home Secretary David Blunkett said.
Blunkett also said the move would have been a surrender to terrorism, reports CBS News Correspondent Steve Holt.
Light tanks and 450 troops were deployed at Heathrow Airport west of London Tuesday, when police warned that Osama bin Laden's al Qaeda network might try attacking the British capital.
Police also added patrols in central London in response to "a potential threat to the capital" amid terrorism fears ahead of a possible war on Iraq.
Beginning Monday night, police randomly stopped cars for inspection.
London Mayor Ken Livingstone said that any attack against the capital would probably involve a small device.
Police didn't specify the nature of the threat or how long the operation would last, but warned the al Qaeda group might try attacking London at the end of this week's Muslim festival of Eid al-Adha.
Prime Minister Tony Blair personally authorized the security operation, his spokesman said Tuesday, adding that the measures were being taken in response to a "specific threat."
Britons are used to seeing more security than Americans — past bombing campaigns by the Irish Republican Army led to the removal of most trash canisters from downtown London.
London is also home to a small community of Islamic radicals. Shoe bomber Richard Reid and suspected 20th Sept. 11 hijacker Zacarias Moussaoui worshipped at the same mosque in north London.
Sheikh Omar Bakri Mohammed, a leader of the small Islamic group al-Muhajiroun, said Tuesday that al Qaeda supporters in Britain might carry out attacks.
Police in Britain have arrested several terrorist suspects since the Jan. 5 discovery of the deadly poison ricin in a north London apartment. Reports have indicated a suspicion that the ricin was going to be used to poison food at a nearby military installation.
In a report to the Security Council last week, Secretary of State Colin Powell linked the apparent ricin plot to alleged al Qaeda operative Abu Musab Zarqawi, whom the Bush administration claims Saddam Hussein has given refuge.
Attorney General John Ashcroft mentioned the ricin plot in his explanation of the increase in the terror alert system from "elevated" to "high" last week.
In testimony to the Senate Intelligence Committee Wednesday, the director of the CIA warned al Qaeda may have developed a dirty radiological bomb and could be planning an attack for later this week.
However, George Tenet did not mention any threat to Britain, but instead said he saw a risk in the United States and on the Arabian Peninsula.
Also Tuesday, the al-Jazeera television network played a new audiotape apparently from bin Laden in which he urges measures against Americans and "those that help America." Britain is Washington's main ally in its confrontation with Iraq.
John Reid, chairman of the governing Labor Party, said the world faced a terrorist threat on the scale of the Sept. 11 attacks, but later said he did not mean that the threat at Heathrow was of that magnitude.
"This is about a threat of the nature that massacred thousands of people in New York," Reid said during a visit to Manchester, responding to a question about the security measures in the London area.
But Reid said later that he was referring to the overall phenomenon of international terrorism, not to a specific threat against Britain.