British officials say they have disrupted a major terrorist plot to blow up passenger aircraft in flight, prompting authorities to dramatically step up airport security.

A statement released Thursday in London by Scotland Yard says the plot involved explosives hidden in hand baggage and that the targets were planes flying from Britain to the United States.

British Home Secretary John Reid says those involved in the terror plot intended to bring down several aircraft through mid-flight explosions to cause considerable loss of life.

Washington has raised its terrorist threat alert to "severe", the highest level, for commercial flights bound for the U.S. from Britain. The threat level for all other commercial flights through U.S. territory has been raised to "high".

Britain has raised its national public security threat level to "critical" -- the highest category, which means an attack is expected imminently.

Authorities have dramatically stepped up security measures at British airports, causing major delays. German airline Lufthansa has canceled all flights to Britain.

Passengers cannot bring any hand baggage with them on planes departing from any airport in Britain. Authorities are allowing passengers to board flights carrying only bare essentials, such as passports and wallets.

British Home Secretary Reid says the heightened security measures are designed to allow air travel to continue as much as possible.

British police say anti-terrorism squads made a series of arrests in the London area overnight after an intelligence operation lasting several months. News reports in London say at least 18 people were arrested.

Some information for this report was provided by AFP, AP and Reuters.