Preparations for the London Olympics have put Britain’s intelligence agencies under significant pressure, as the country stages its largest ever peacetime security operation, British lawmakers said on Thursday.

In its annual review, parliament’s Intelligence and Security Committee said al Qaeda and its affiliated groups, along with armed groups opposed to the peace deal in Northern Ireland, still posed the most significant threat to Britain.

But the committee, which provides oversight of 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,” said committee chairman Malcolm Rifkind.

However intense work for the Olympic Games, which start on July 27, have put a strain on the work of domestic intelligence agency MI5, and was a “critical challenge for all concerned”.

The review said the “Arab Spring” revolutions across the Arab world had dominated the work of Britain’s security agencies in 2011, adding the developments had caught many within the intelligence community by surprise.

“We appreciate that it is often impossible to predict such events,” said Rifkind.

“However there remains a question as to whether, once events began to unfold, the agencies should have anticipated the possibility that the unrest would spread quickly across the region.”

Last month, MI5 chief Jonathan Evans warned in his first public speech for two years that al Qaeda militants were using countries which toppled leaders during the Arab Spring as bases to recruit Western youths for attacks on Britain.