A major terrorist plot to blow up 20 planes on their way from the U.K. to the U.S. has been thwarted according to British officials.

Britain’s Home Secretary John Reid said the alleged plot was significant and that terrorists aimed to “bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life.”

Police arrested a number of people in London after a major covert counterterrorism operation that had lasted several months, but did not immediatley say how many.

Passengers faced delays as tighter security was hastily enforced at the country’s airports and additional measures were put in place for all flights. British Airways said laptop computers, mobile phones, iPods, and remote controls were among the items banned from being carried on board.

The U.S. government raised its threat level to the highest level for commercial flights from Britain to the United States early Thursday and also stepped up the general level for all flights within the U.S. or entering the country.