Security at London's air terminals is at an all-time high following the arrest of a man with a grenade at Gatwick airport on Thursday afternoon.

Police said that the man was a Venezuelan who travelled to Britain on a flight from Bogota.

Two others have also been arrested at Heathrow airport in a separate police operation and an access road to Stansted airport, North London, has been sealed off.

The arrests have come after a massive deployment of troops, armoured cars and helicopters at Heathrow.

British intelligence reports have warned of an Al Qaeda plot to smuggle shoulder-held Sam-7 missiles into the UK with the aim of downing a passenger aircraft.

The missiles are said to be part of a consignment of weapons that went missing in Afghanistan some 20 years ago.

After intelligence agents alerted Scotland Yard, police anti-terrorist experts immediately asked for and received troops to patrol the outskirts of Heathrow.

British Home secretary David Blunkett confirmed in parliament on Thursday that a Royal Air Force Nimrod reconnaissance aircraft has been deployed to patrol the skies above London.

Asked to explain the additional airport security, Blunkett said in an emergency statement to the House of Commons: "I do not believe it is responsible to provide a running public commentary from the despatch box on every turn and development in the operational measures taken to protect the country's security.

Blunkett earlier confirmed that the government had considered shutting down Heathrow altogether.

After talks today with visiting Australian Prime Minister John Howard, British Premier Tony Blair said, "We cannot and should not start disclosing details of everything we know or may know. Its important we take every precaution we can in order to keep people safe."