Jack Straw said UK security would be heightened

Foreign Secretary Jack Straw has already warned that British people could now expect to see security such as that seen in Northern Ireland being implemented across the whole country.

All government buildings, military bases and financial institutions have been placed on a higher state of alert.

American airbases in the UK, including those at Lakenheath and Mildenhall in Suffolk, have tightened their security.

British people abroad have been advised to take extra precautions over their personal safety.

Airport security

Transport Secretary Stephen Byers told the Cabinet on Thursday that security would remain tight at airports.

The 6,000ft government no-fly zone over central London, imposed on Tuesday, will not be lifted until midnight on Saturday.

London's City airport will re-open on Friday but with planes diverted away from central London on their flight paths.

Restrictions on light aircraft and helicopter flights in the UK have been lifted.

The prime minister has already announced increased security at all government buildings and military premises and the police across the whole of the UK are on full alert.

He also said advice had been given to major financial and business institutions about appropriate security measures.

A spokesperson for the British Airports Authority (BAA) said: "As soon as we became aware of the situation we put our airports on full alert and liased with the Department Transport, Local government and the Regions (DTLR) about further security measures, which have been put in place.

American airspace reopened on Thursday afternoon.

But officials have warned that it could be some time yet before passenger services to the US fully resume.

The first transatlantic flights since Tuesday are set to leave Heathrow on Thursday night.