Police were today continuing to question 23 suspects about the alleged terror plot to blow up nine transatlantic airlines.

As the threat of a terror attack in Britain was downgraded from critical to severe by the Home Office, detectives and forensic officers carried on their investigations at locations in East London, Birmingham and High Wycombe, Buckinghamshire.

Meanwhile Sir Ian Blair, the Metropolitan Police Commissioner, warned of the threat facing Britons by terrorists who he said wanted to bring "mass murder on an unimaginable scale."

Mr Blair said that police had been watching the group of people arrested last week "for some time" and had made the decision to arrest them after deciding a threat to the public was imminent.

"What we always have to do is balance waiting to gather more evidence and make sure you get all the people, against the risk to the public by not moving in earlier," said Mr Blair.

"That’s the decision that was reached last Wednesday evening. There’s a point where the information reaches a level of concern that means if you don’t take action it is indefensible," he said.

This afternoon police in High Wycombe continued to search King’s Wood in the north of the town for more clues.

Officers guarded the entrance to the wood - which they first started combing on Thursday morning - and were stopping dog walkers and any other members of the public from entering the area.

The Met has already set up their own portable buildings in the car park of the Dolphin pub and throughout the morning scores of police officers in blue boiler suits and baseball caps entered the wood with shovels, pick axes and sticks to scour the undergrowth.

In Walthamstow, East London, a three storey house on Forest Road was still cordoned off and a wooden and tarpaulin construction built to cover the front door as police officers continued their search for clues.

Police are also reported to have discovered guns and £19,000 cash during a search of other properties raided in Walthamstow on Thursday night.

In Pakistan up to 17 others are believed to be in custody including Rashid Rauf - a Briton who has been officially described as a "key" suspect and is thought to be the brother of Tayib Rauf, 22, one of those arrested in Birmingham last week.

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