Syria's arsenal of chemical weapons could fall into the hands of al Qaida militants if president Bashar Assad is toppled 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 there was "serious concern" about the security of the "vast stockpiles" of chemical weapons amassed by the regime.
In its annual report, the committee - made up of senior MPs and peers - said the list of chemical agents acquired by Assad was thought to include sarin, ricin, mustard gas and VX - described as "the deadliest nerve agent ever created".
The danger was underlined by MI6 chief Sir John Sawers who told the committee there was the risk of "a highly worrying proliferation around the time of the regime fall".
The ISC 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 Qaida-linked elements fighting the regime had already attempted to acquire chemical weapons for probable use in Syria. Britain has also accused Assad of using chemical weapons against his own people.
The threat was underlined by Foreign Secretary William Hague who told the Commons that Britain was considering supplying the Syrian opposition with protective equipment against chemical and biological weapons use.
In its report, the ISC said that al Qaida elements and other jihadists fighting in Syria now represented "the most worrying emerging terrorist threat" to the UK and its allies, amid fears they could take advantage of the "permissive environment" 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," it 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."