LONDON - Authorities in London thwarted a terrorist plot to blow up aircraft mid-flight between Britain and the U.S. using explosives smuggled in hand luggage, police said Thursday.

In a file phtoo a jet takes off from London's Heathrow Airport Tuesday June 6, 2006. Authorities in London thwarted a terrorist plot to blow up an aircraft in mid-flight between Britain and the U.S. using explosives smuggled in hand luggage, police said Thursday Aug. 10, 2006. [AP]

Britain's Home Secretary John Reid deemed the alleged plot significant and said it was designed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."

The United States government has raised its security threat level in response to a reported terrorist plot in Britain.

Authorities arrested a number of people overnight in London after a covert counter-terrorist operation lasting several months, police said in a statement.

The national threat level was raised to critical - a warning level that indicates the likelihood of an imminent terrorist attack. The threat rating, based on a U.S. Department of Homeland Security's model was posted on the Web site of Britain's MI5, the British domestic spy agency.

Security at the country's airports was increased and additional measures were put in place for all flights.

British Airways advised all passengers that they would not be permitted to carry any hand baggage on board any aircraft departing from any airport in the country.