British Police are continuing to hold 24 people they believe were involved in a foiled terrorist plot to blow up a number of transatlantic passenger planes.
The arrests sparked a major alert that grounded flights in and out of Britain and caused air traffic chaos around the world.
There are suspicions that there are strong al-Qaeda links to the attacks.
More than 30 flights between Ireland and Britain were disrupted.
Although security still remains very tight, flights from this country to Britain are returning to normal though some delays can be expected tomorrow.
The White House has described the plot as a direct threat to the US.
It is thought the plot involved explosives being smuggled on board aircraft hidden in hand luggage.
Most of the 24 arrests were in the London and Thames Valley areas.
A US Homeland Security official said tonight that all of those arrested by British police today were British citizens.
The security threat level in the US for commercial flights arriving from the UK remains at 'severe'.
US President George W Bush has said that while the US is safer than before the 11 September attacks, it was still not completely safe and it would be a mistake to believe there was no threat to the United States.
He said that the events in Britain today were a stark reminder that the United States was at war with Islamic fascists.
He paid tribute to the British government and the cooperation between the two countries on matters of security.
Operation was 'international in scope'
US Homeland Security Secretary, Michael Chertoff, said that, while the operation centered on Britain, it was international in scope.
He said the plot appeared to have been well planned and well advanced.
The US is sending air marshals to the UK to provide extra security on transatlantic flights.
It is believed the plot targeted three US airlines.
Britain's Home Secretary, John Reid, said that had the attacks succeeded, there would have been an unprecedented loss of life.
Speaking outside Scotland Yard, Deputy Commissioner Paul Stephenson of the Metropolitan Police said the intention had been mass murder.
Further flight disruptions to and from the UK can be expected for the rest of the day.