A team of suspected terrorists involved in an alleged UK plot to blow up trans-atlantic airliners escaped capture because of interference by the United States, The Independent has been told by counter-terrorism sources.

An investigation by MI5 and Scotland Yard into an alleged plan to smuggle explosive devices on up to 10 passenger jets was jeopardised in August, when the US put pressure on authorities in Pakistan to arrest a suspect allegedly linked to the airliner plot.

As a direct result of the surprise detention of the suspect, British police and MI5 were forced to rush forward plans to arrest an alleged UK gang accused of plotting to destroy the airliners. But a second group of suspected terrorists allegedly linked to the first evaded capture and is still at large, according to security sources.

The escape of the second group is said to be the reason why the UK was kept at its highest level - "critical" - for three days before it was decided that the plotters no longer posed an imminent threat.

The alleged airliner plot caused chaos and fear at airports throughout Britain when details emerged in August of an alleged plan to smuggle liquid explosives on board up to 10 flights and destroy them after take-off.

As a result of the alert, airports banned passengers from carrying liquids in their hand luggage and imposed tough new security checks.

The operation was one of the largest undertaken by the police and MI5, yet two counter-terrorism sources suggested that the intervention of the Americans was due to "inexperience and naivety" and that they were after a "short-term success".

American intelligence chiefs are understood to have persuaded the Pakistani authorities to arrest a British citizen, called Rashid Rauf, on Wednesday 9 August this year.

Mr Rauf was suspected of being closely linked to group of men and women in Britain who were allegedly involved in the airliner terror plot, and the arrest prompted emergency meetings involving ministers, police and intelligence chiefs - who were still investigating the case.

Assistant Commissioner Andy Hayman, who has overall responsibility for the Metropolitan Police's anti-terrorist investigations, was forced to abandon a family holiday in Spain and catch an easyJet flight to Britain, arriving at 3am on Thursday 10 August.

Fearful that the arrest might tip off the alleged plotters, Scotland Yard, in consultation with MI5, decided to act and sanctioned a series of raids in the early hours of Thursday. The police arrested 24 men and women and later charged 13 in connection with the alleged terror plan. Britain has requested the extradition of Mr Rauf.

Scotland Yard has declined to comment about any other groupings who may have been involved in the alleged plot.

John Reid, the Home Secretary, downgraded the threat after three days from "critical" to "severe", meaning an attack was "highly likely", but not imminent.

It is understood that one of the main reasons why the highest level of alert was maintained was that authorities were trying to establish whether the second group was capable of an attack. After they satisfied themselves that it did not have the equipment to launch an assault, the threat level was downgraded.

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