Britain's high profile as an ally of the US has raised fears that it could be the next target for a terror attack by Islamic extremists.

Concern has been increased by reports about radical Islamic activity in the UK and evidence that some of the men identified as suicide hijackers in the US attacks passed through Britain. "They're here," warned a headline in the Sunday Express newspaper, referring to four arrests made in the UK last week in connection with the attacks.

The resolve and energy shown by Tony Blair, prime minister, in helping to rally an international coalition behind the US has strengthened his domestic standing. But many are preoccupied by what Mr Blair - like President George W. Bush - has called a "war".

Several newspapers have exercised their imagination about a chemical or germ warfare attack on London, which Geoff Hoon, UK defence secretary, said was "certainly something we have to guard against".

A company selling army surplus equipment on the internet from Worcestershire in central England has exhausted its stocks of gas masks and protective suits.

As in other main European cities, policing has been stepped up in London, already on guard against attack by Irish terrorists. Water and electricity groups have been told to tighten security, local authorities to upgrade civil defence plans, and hospitals to ensure they are prepared for a "mass casualty incident".

A number of government and other buildings set for a "London Open House" project at the weekend remained closed.

It is against this background that debate has been revived about compulsory national identity documents, which the government is considering as part of a package of anti-terrorism measures. The absence of identity cards - standard in most of continental Europe but not in the UK, Ireland or the Nordic countries - has been seen as a symbol of civil liberties. Registration cards were introduced during the second world war and abolished in 1952.

Mohammed Sarwar, the first Muslim member of parliament, who belongs to the governing Labour party, warned that the UK risked being seen in the Islamic world as the "yes-person" of the US by supporting armed retaliation.

"If Britain sides with America," he said, "then there is the danger terrorists will target Britain."

Sir John Stevens, chief of London's Metropolitan Police, who is in charge of anti-terrorism operations, fuelled concern last week when he said: "Who are the biggest allies of America? Which is the next biggest target? It's got to be here."

But other security experts questioned whether Britain would be an obvious target. Its longstanding connections with countries such as Pakistan meant that it tended to be viewed more sympathetically than the US.

As in the US, the public mood has stoked fears of reprisals against members of Britain's 1.5m-strong Muslim community, mainly children of migrants who arrived more than 30 years ago. One Muslim, a retired British government official whose daughter was feared killed in the World Trade Center, said he had faced "gnashed teeth, bad looks" from people in the street.

Last Friday, a delegation of Muslim leaders met ministers to discuss the government's response to what some groups called "a wave of anti-Muslim hysteria".

But Mohammed Abdul Zaki Badawi, principal of the Muslim College in West London and a leading moderate voice, said these concerns had given way to apprehension about the consequences of a US-led military action.

"Things are beginning to quieten down," he said. He complained about the publicity news media were giving to radical groups operating in Britain such as Al-Muhajiroun (The Emigrants), which seeks a worldwide Islamic state. Reports have focused particularly on extremists offering military training courses for young Muslims.

"They're a very, very small group, and insignificant within the community," he said. "They are taken too seriously. This is a tragedy for us."

Dr Badawi is chairman of the Imams and Mosques Council, which called for condemnation of the attacks against New York and Washington and sympathy towards the US. Only two of more than 1,000 British mosques had defied the directive, he said.

He did not believe there would be any attempt at disruption in protest against British involvement in a US-led action, although Muslims, like many others in Britain, were concerned about the scale of the military response. "We may disagree with the government, but when it comes to the crunch we are British."