London was today on red-alert amid fears terrorist fanatics would target Britain - America's biggest ally.

Prime Minister Tony Blair returned to London after cancelling his keynote speech to the TUC conference in Brighton to chair a snap Cabinet meeting.

He has called together the emergency "Cobra" committee at 10 Downing Street.

Foreign Secretary Jack Straw, Home Secretary David Blunkett and Defence Secretary Geoff Hoon were summoned to attend.

Mr Straw described today's attacks in America as "an attack on civilisation".

The London Stock Exchange was evacuated, as was Canary Wharf in east London - one of the tallest buildings in Europe.

Security has been stepped up at all public buildings in the capital.

All flights from Britain to America have been cancelled.

A Heathrow airport spokesman said: "American airspace has been closed and flights to America have been grounded.

"There is disruption to flights to America and we expect substantial delays at Heathrow.

"Passengers due to fly to America are advised to contact their airlines."

Planes already bound for America which were unable to turn back were being allowed to land in Canada.