"Since July, the pace of investigation, the scale of the threat has not diminished in any way whatsoever," Peter Clarke, the national coordinator of terrorism investigations, told a security conference in Germany.

He added: "There are no terrorist-free zones in the United Kingdom. The footprint of international terrorism is in every part of the country."

Clarke declined to give a progress update on the London bomb probe, which he said involved more than 35,000 documents, 10,000 witness statements, 38,000 police exhibits and 90,000 pieces of computer evidence.

Asked about the extent of foreign backing and support for the four young Muslim men who blew themselves up on the London transport network on July 7, 2005, killing 52 people, Clarke told Reuters: "That's something we're still looking at."

Three of the attackers who carried out Western Europe's first suicide bombing were British-born men of Pakistani origin, and the fourth was born in Jamaica.

Several of the plotters had travelled to Pakistan before the bombings, but police have not said publicly if they believe they received training, support or instructions from there.

Clarke said he had just returned from a trip to Pakistan but this was to discuss broader cooperation issues, not specifically the July 7 investigation.

INTELLIGENCE

Commenting on the wider threat from Islamist radicals, he told the conference he was concerned about the difficulty of getting sources inside the Muslim community to come forward with intelligence.

"Most of the cases that we have in the United Kingdom are as a result of intelligence that has come from overseas, or from technical means. There is not the wealth of intelligence that I would like coming from within the communities," Clarke said.

"We must do more to build our links into the Muslim communities, so that those who wish to reject extremism and expel the extremists and give information about them can have the confidence to do so."

Clarke said several "hugely important" terrorism trials in Britain this year would present a test for the courts and judicial system.

"These are cases, the like of which have never come before the British courts before. The international dimension, the sources of evidence, the types of evidence ... will not have been tested before in our courts," he said.

"It will be a real test to see whether our system, and the rules of evidence, are capable of dealing with these issues."

In one of the cases, described by police as the country's biggest terrorism trial since the September 11 attacks on the United States in 2001, seven Britons went on trial last month charged with plotting to carry out bomb attacks.

Six of them were arrested in 2004 during raids in which police found 600 kg (1,320 lb) of ammonium nitrate, which can be used to make bombs.