Britain's intelligence agencies have come under intense pressure as the country stages its largest ever peacetime security operation ahead of Olympic Games in London, legislators have said.
The disclosure was made on Thursday amid anger over massive presence of security forces on the streets.
But parliament's intelligence and security committee defended the deployment, saying in its annual review that al-Qaeda and its affiliated groups, along with armed groups opposed to the peace deal in Northern Ireland, still posed significant security threats to the UK.
The committee, which oversees intelligence agencies, said successful counter-terrorism work was paying dividends.
"The Security Service considers there are signs for cautious optimism, with the number of attacks on national security targets falling in 2011," Malcolm Rifkind, the committee's chairman, said.
But he added that intense preparations for the Olympic Games, which start on July 27, had put a strain on the work of domestic intelligence agency MI5 and posed a "critical challenge for all concerned".
Last month, MI5 chief Jonathan Evans said in his first public speech for two years that al Qaeda militants were using countries which toppled dictators during the Arab Spring as bases to train radical British youths for attacks on Britain.
"The Security Service has reprioritised its work to enable them to counter potential threats from al Qaeda and its affiliates; (Irish) Republican dissidents; hostile states and others in the run-up to or during the Games," Rifkind said.