The threat levels were raised over an alleged plot to bomb passenger jets flying from Britain to the United States.
The U.S. Department of Homeland Security said it was reducing the threat level for commercial flights from Britain to the United States from "red" to "orange," or to a high threat level.
Britain's domestic intelligence service, MI-5, said the country's security threat level had been lowered from "critical" to "severe," meaning that authorities believe a terrorist attack is still "highly likely."
British Home Secretary John Reid said in a statement that authorities believe that the main suspects in the alleged plot were arrested last week.
More than 20 people suspected of involvement in the alleged plot were arrested in Britain last week.
(compiled from agency reports)
