Britain on Monday lowered its security threat level to "severe," after raising it to "critical" amid Thursday's foiling of an attempt to blast U.S.-bound planes, according to the website of intelligence service MI-5. 

The foiled terror plot, in which suspects planned to smuggle liquid explosives aboard passenger jets, has prompted the British government to upgrade the threat level to the highest of the five-level system and caused major airports worldwide to tighten up security.

According to MI-5, the level "severe" means an attack is highly likely and indicates a continuing high level of threat to Britain, while "critical" warns of an imminent attack.

Downgrading the level allows passengers to carry one item of hand baggage through the airport check point. However, the ban on liquids remains, with few exceptions including liquid prescription medicine and baby milk, with proven genuineness.

"The police believe that the main suspects in the alleged plot were arrested last week," said Home Secretary John Reid.

"There is still a very serious threat of an attack. The threat level is at severe indicating the high likelihood of an attempted terrorist attack at some stage, and I urge the public to remain vigilant," he added.

(Xinhua News Agency August 14, 2006)