DRASTIC security measures were in force across Britain today following the devastating terrorist attacks in the US.

Government buildings, military bases and financial institutions were on red alert after yesterday's atrocities at the World Trade Centre and the Pentagon.

Prime Minister Tony Blair announced sweeping security controls in an attempt to prevent similar violence in the UK.

Major airports are on heightened guard against the threat of terrorist action and all flights to America have been cancelled.

Other flights can leave Britain only if maximum security is guaranteed and aircraft are banned from crossing central London.

Extra police officers have been drafted in and "all necessary precautions" are being taken to prevent similar attacks in the UK, Metropolitan Police Commissioner Sir John Stevens said.

Security has been stepped up at military bases across the UK. The MoD said it was closely monitoring the situation and would take any extra steps it "deemed necessary".

The Prime Minister said Britain will stand full square alongside the US in the battle to drive the "new evil" of terrorism from the world.

"We 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.

"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."

The Government has been in contact with US, European and other allies, and is co-operating with them on issues of security.

Mr Blair has spoken to Chancellor Gerhard Schroeder of Germany and President Jacques Chirac of France about the tragedy and plans to speak to President Bush at the earliest opportunity.

Meanwhile, the Foreign Office has warned Britons abroad to be on alert for possible "heightened tension and disruption worldwide."

"All British nationals overseas should exercise caution and avoid situations where there might be tension and stay in touch with fast moving events," the FO said in a statement.

"There may be demonstrations and violence, particularly in Muslim countries."

It also warned of a "strong risk of further atrocities" in the US.