Civil aircraft are being prohibited from crossing over central London, Tony Blair said on Tuesday night, as he announced security measures in response to the terrorist attacks on the US.

Speaking in Downing Street after a meeting of Cobra, the cabinet's emergency committee, he said Britain would stand "shoulder to shoulder" with the US in the battle to drive the "new evil" of terrorism from the world.

Security at airports had been stepped up to the highest levels. No flights would take off from the UK "for which we cannot apply the highest standards of security to aircrew and passengers".

Private flights had been stopped except where specially authorised. Flight paths had been changed so there would be no civil flights over the capital.

Security had been increased in government buildings and military premises, and police across the UK had been put on full alert.

Defence facilities around the world had been moved to high alert, to ensure protection of British service personnel. Financial and business institutions were being advised on security.

Other security measures had been taken and Britain was in touch with US, European and other allies, he said. Cobra will meet again at 8am today.

"Obviously, some of these measures, not least the effect upon airports, will lead to disruption. But other than the specific measures we have taken, or advised others to take, I believe business and everyday life can continue as normal," Mr Blair said.

He added: "This is not a battle between the US and terrorism, but between the free and democratic world and terrorism.

"We therefore, here in Britain, stand shoulder to shoulder with our American friends in this hour of tragedy and we, like them, will not rest until this evil is driven from our world."

Of the terrorists, Mr Blair said: "As for those that carried out these acts, there are no adequate words of condemnation. Their barbarism will stand as their shame for all eternity."

Mr Blair said: "It is hard even to contemplate the utter carnage and terror that has engulfed so many innocent people. We have offered to President Bush and the American people our solidarity, our profound sympathy and our prayers."

The Cobra meeting was attended by Jack Straw, foreign secretary, David Blunkett, home secretary, Gordon Brown, chancellor, Geoff Hoon, defence secretary, Stephen Byers, transport secretary, and Alan Milburn, health secretary, as well as military chiefs.

Cobra - which stands for Cabinet Office Briefing Room A, where the committee meets - is pressed into action during emergencies.

Most recently it convened at the end of March to tackle the foot-and-mouth outbreak. Ministers meet in the windowless, white-walled Cabinet Office basement bunker, which is linked to 10 Downing Street.

The US catastrophe presented the first challenge for the 115-strong civil contingencies secretariat, which was set up in the cabinet office over the summer. It is responsible for drawing up a strategy for handling disasters and co-ordinating departments.

It was established after the prime minister became frustrated at the way in which the two crises of his administration - the fuel protest and foot-and-mouth - were handled.