Specialist police search teams were scouring an area of woodland west of London tonight, hunting for clues in the investigation of an alleged plot to blow up as many as a dozen American passenger airlines over the Atlantic.

The search was part of a huge operation that saw a number of addresses raided throughout the night in High Wycombe, Buckinghamshire, as well as in Birmingham and East London. Twenty one people were arrested and British airports ground to a halt because of the tightened security.

The arrests were carried out with the help of the Pakistani intelligence service, which had been working closely with MI5 and Scotland Yard to foil the alleged plot. Many of those detained were said to hold dual British and Pakistani nationality and are believed to have travelled to Pakistan frequently.

British authorities had sought cooperation from Pakistani officials a few months ago and the information provided was crucial in thwarting the attacks, according to a highly placed Pakistani security source.

As the police searched a large swathe of High Wycombe's King’s Wood, a nearby Victorian house was being guarded by police officers. It is understood that the property was also raided overnight by anti-terror squads.

Neighbours of another house raided by police, a three-bedroomed ex-council house, said that an Asian family had lived there for around 15 years, although they had kept themselves to themselves.

Two other properties in the area were also raided overnight and earlier today police evacuated people from nine homes around Walton Drive - although they were keen not to provoke alarm.

"The decision to evacuate nine houses in the Walton Drive area earlier today is standard practice in these cases and purely a precautionary measure. There is no need for residents to be unduly alarmed," said Superintendent Mike Ismay of Thames Valley Police.

"There is no direct intelligence at this time to suggest that the threat to people living in the Wycombe area is any greater than normal, although we would urge members of the public to be vigilant and report any suspicious activity to us."

There was no indication as to what the police search teams were looking for or whether any of the properties involved were thought to be bomb factories. US security officials, more forthcoming than their British counterparts, suggested that the terror plot had involved liquid explosives carried in hand baggage.

Five properties in Walthamstow, East London were also raided.

Residents said that at 10.30pm last night they saw around twenty officers burst into a rundown three-storey building that is believed to contain three flats.

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