The shortcomings mean the Government Communications Headquarters (GCHQ), the country's intelligence monitoring hub, cannot adequately support the security services in the fight against terrorism, the report said.

Britain's police and security services disrupted an alleged plot to bring down airliners in the mid-Atlantic in 2006 and a plan to abduct a British Muslim soldier early last year, but the report by the influential Intelligence and Security Committee said the wealth of spying demand was taking its toll.

'The substantial growth in the number of security service counter-terrorism operations in recent years has made it difficult for GCHQ to maintain high-quality support to an ever-growing number of investigations,' the committee said.

The report - compiled after hearing evidence from the heads of GCHQ, the Secret Intelligence Service (SIS) and the security services - said about 200 extremist networks were being probed.

'Some of which have both the intent and capability to carry out attacks against the UK or UK interests overseas. There will still be others about which little or nothing is known,' the cross-party committee concluded.

Stretched
David Pepper, director of GCHQ, told the committee his listening centre was stretched.

'By the end of the year we had found we were managing to support most of security service's highest priority operations, but we were not achieving the quality of support that we and they had agreed we should aim for ... essentially because we were spreading ourselves too thin,' he said in his evidence.

'We do not have enough CT (counter-terrorism) resources.'

The lawmakers, who said Britain faced a 'serious and sustained' threat from groups such as al Qaeda, promised to monitor future funding for GCHQ.

The lawmakers also said the leaking of information on counter-terrorism operations to the media was putting lives at risk and jeopardising operations.

Britain's security and spy services have been on high alert for attacks or security threats since suicide bombers killed 52 people on London's transport system in July 2005.

Prosecutors have launched a series of cases against suspected plotters, or those who have used the Internet to plan attacks or solicit others to carry them out.

Convictions have been secured in many plots, including a plan to detonate a fertiliser bomb in 2004 and the attempted bomb attacks on London's transport system on July 21, 2005. -- REUTERS