The UK has been placed on its highest level of terrorism alert after an attack on Glasgow Airport was linked to two car bombs found in London, BBC reported.

The decision to raise the threat level to "critical" means that an attack is imminent.

Ministers, police and security service officials held their third meeting of Cobra in recent days on Saturday evening, this time with a video link to ministers in Scotland.

Prime Minister Gordon Brown urged the public to remain vigilant.

"I know the British people will stand together, united and resolute," he said.

In a televised statement, Brown thanked the police, the security and emergency services for the "dedicated professionalism" that has been shown in London and Glasgow Airport.

"The first duty of the government is the security and safety of all the British people, so it is right to raise the level of security at airports and in crowded places in the light of the heightened threat," he said.

The critical threat level was also in place after the 7 July 2005 suicide bomb attacks on London.

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