By Adrian Croft

LONDON, April 24 (Reuters) - London police's anti-terrorist chief said on Tuesday it was a "sensible assumption" that Islamist militants will strike again in Britain.

Reviewing British counter-terrorism efforts since the Sept. 11 attacks on U.S. cities, Peter Clarke said the strategic threat from Islamist militants was "enduring and to a significant extent targeted at the United Kingdom".

"Within the country we have people who are sympathetic to the terrorist cause and prepared to carry out attacks against their fellow citizens," he said in a lecture.

The police and security service had stopped a number of attacks in Britain and more than 100 people were awaiting trial on terrorist-related charges, he said.

"Nevertheless, we suffered the appalling attacks of July 2005 and the only sensible assumption is that we shall be attacked again," he said.

On July 7, 2005, four British Islamist suicide bombers blew themselves up on London's transport network, killing 52 people.

Clarke said he did not disagree with a figure given by Elizabeth Manningham-Buller, former head of the British intelligence agency MI5, who said in November that her agents were tracking some 1,600 suspected Islamist militants.

But he said he stood by a comment he made last year that the total number may run into thousands.

Clarke said an alleged plot by Islamist militants to blow up transatlantic airliners, foiled by police last August, was another step in what appeared to be a "trend towards more ambitious and more destructive attack planning".

MOMENTUM

"The extremists have a momentum that must be stopped," he added.

In the 2005 London attacks and in other cases, police had spotted a trend for groups of British citizens to travel to Pakistan for training and then returning to Britain and building up their networks in preparation for launching attacks, he said.

Clarke said the threat from al Qaeda was very different from the threat Britain faced for 30 years from Irish Republican Army (IRA) guerrillas opposed to British rule in Northern Ireland.

The al Qaeda networks are large, mobile and resilient, he said.

"We have seen how al Qaeda has been able to survive a prolonged, multinational assault on its structures, personnel and logistics. It has certainly retained its ability to deliver centrally directed attacks here in the UK," he said.

"Arrested leaders or key players are quickly replaced, and disrupted networks will re-form quickly. Suicide has been a frequent feature of attack planning and delivery," he said.

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