Al-Qa'ida elements fighting with the rebels in Syria could gain access to the regime's stockpiles of chemical weapons with potentially “catastrophic” consequences, British spy chiefs have warned.
The parliamentary Intelligence and Security Committee (ISC), which oversees the work of the intelligence agencies, said extremist elements in Syria were assessed to represent "the most worrying terrorist threat" to the UK and its allies.
In its annual report, the committee said there was "serious concern" about the security of the "vast stockpiles" of chemical weapons amassed by the regime of President Bashar al-Assad.
They are thought to include sarin, ricin, mustard gas and VX - described as "the deadliest nerve agent ever created".
MI6 chief Sir John Sawers told the committee there was the risk of "a highly worrying proliferation around the time of the regime fall".
The committee said: "There has to be a significant risk that some of the country's chemical weapons stockpile could fall into the hands of those with links to terrorism, in Syria or elsewhere in the region. If this happens, the consequences could be catastrophic."
Prime Minister David Cameron disclosed last month that al-Qa'ida-linked elements in the opposition movement had attempted to acquire chemical weapons for probable use in Syria.
The ISC said there is now a risk that extremist elements in Syria could take advantage of the "permissive environment" there to plot attacks on targets in the West.
"Large numbers of radicalised individuals have been attracted to the country, including significant numbers from the UK and Europe," the report said.
"They are likely to acquire expertise and experience which could significantly increase the threat posed when they return home.
"Furthermore, there is growing concern about the risks around extremist groups in Syria gaining access to regime stocks of chemical weapons."
The report also highlighted the growing threat of attacks by "lone actors", such as the stabbing of Labour MP Stephen Timms while holding a surgery in his east London constituency in 2010.
It said the risks are inherently much more difficult for the security services to manage as, by their nature, lone actors are much harder to detect - something al-Qa'ida appears determined to exploit.
One Home Office official told the committee: "There is no doubt that the more sophisticated people in al-Qa'ida recognise that groups are, in some ways, a thing of the past; and that encouraging lone acts of terror is exactly the way forward."
The ISC also expressed concern that the agencies were struggling to find projected efficiency savings due to be completed by 2014-15 - potentially putting frontline services at risk.
It highlighted a forecast £59 million shortfall in the £220 million savings that the agencies - MI5, MI6 and GCHQ - were supposed to have achieved through more collaborative working.
While the agencies said they were "fairly confident" that the targets set by the Treasury would be met, the committee said it "does not fully share that confidence".
"We recognise that, during the run-up to the Olympics, operational requirements were, rightly, prioritised over efficiency savings but time is running out - we are already over halfway through the spending review period in which these savings must be found," it said.
"It is essential that real and sustainable efficiencies are delivered if frontline capabilities are to be protected. More needs to be done urgently."
The committee also highlighted the threat to the UK of cyber attacks which GCHQ said is "at its highest level ever and expected to rise further still".
While foreign states continue to pose the greatest threat - the report highlighted the alleged involvement of China and Russia - the committee said there is evidence of some countries turning to private groups to carry out state-sponsored attacks.
"These state-affiliated groups consist of skilled cyber professionals, undertaking attacks on diverse targets such as financial institutions and energy companies," the report said.
"These groups pose a threat in their own right, but it is the combination of their capability and the objectives of their state backers which makes them of particular concern."
It said Government departments are being targeted through attacks on industry suppliers which might hold Government data on their systems.
Such cyber espionage has resulted in the theft of Ministry of Defence data, with "both security and financial consequences for the UK".
Cyber spies seeking sensitive commercial and technical information are also said to be increasingly targeting lawyers and accountants who often hold a lot of their clients' data but whose defences against cyber attack may not be as strong.
Alternatively, GCHQ has warned that spies are targeting overseas subsidiaries of British firms "then swimming up the network on to the UK network".
"The threat the UK is facing from cyber attacks is disturbing in its scale and complexity. The theft of intellectual property, personal details and classified information causes significant harm, both financial and non-financial," the ISC said.