LONDON, Jan 7 (Reuters) - The threat of an immediate attack in Britain by al Qaeda-inspired militants has receded after several prosecutions but networks remain active, the head of the country's national intelligence agency was quoted as saying.

British newspapers said the spy chief had commented that Israel's attack on Gaza could inspire further extremism and warned that the economic fallout from the global credit crunch may bring fresh security risks.

In the first interview to newspapers given by a serving head of Britain's MI5 domestic Security Service, Jonathan Evans noted there had been 86 convictions since January 2007 with nearly half pleading guilty.

"This has had a chilling effect on the enthusiasm of the plotters and they are keeping their heads down," he said in comments published on Wednesday.

"There have been fewer cases where terrorists have moved from facilitating and supporting terrorism to attack-planning."

Four British Muslim men carried out suicide bomb attacks on the London transport system in July 2005, killing 52 people.

Britain has been a target for Islamist militants after it joined the United States in invading Afghanistan and Iraq after the Sept. 11, 2001 attacks on the United States.

Evans said there was no cause for complacency. "There is a seedbed of networks of extremism in the UK from where attacks could come."

Britain has never fallen below its second highest threat level of "severe" since first publishing the warning status in August 2006.

It was briefly raised to "critical" on two occasions after attempted car bombings in London and Glasgow in 2007 and after fears of a plot to blow up transatlantic airliners in 2006.

Evans said Israel's attack on Gaza would see "extremists try to radicalise individuals for their own purpose."

Three quarters of al Qaeda and Islamist plots in Britain had a link to Pakistan, he said.

"The strategic intent of the al Qaeda core (based) in Pakistan is to mount attacks in the UK, and their model is to use British nationals or residents to deliver the attacks."

He warned that a decline in economic power in Britain, the United States and continental Europe following the global credit crunch could create new security threats.

"Where there have been watershed moments, there have often been national security implications from that -- a new alignment.

"We have to maintain flexibility and respond to threats. The world will not remain the same."

AlertNet news is provided by