Islamic extremists fighting in Syria pose one of the most worrying threats to Britain and its allies, particularly if they gain access to the regime's stocks of chemical weapons, parliament's intelligence committee said on Wednesday.

In their annual report, lawmakers said the consequences of Al-Qaeda-linked extremists getting hold of President Bashar al-Assad's reported stocks of the sarin and VX nerve agents, ricin and mustard gas would be "catastrophic".

The committee, which oversees the work of Britain's intelligence agencies, also urged the government to speed up efforts to tackle the "disturbing" threat of cyber attacks.

"Al-Qaeda elements and individual jihadists in Syria currently represent the most worrying emerging terrorist threat to the UK and the West," the report says.

It quotes a warning from the head of external intelligence agency MI6, John Sawers, about the risk of "a highly worrying proliferation around the time of regime fall".

And it concludes: "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."

Elsewhere in the report, the committee warns that government pledges to boost Britain's defences against cyber attacks must be brought forward.

"The threat the UK is facing from cyber attacks is disturbing in its scale and complexity," it said.

State actors posed the biggest cyber threat -- "China and Russia, for example, are alleged to be involved in cyber attacks" -- but it said a number of countries were also thought to be using private groups to carry out state-sponsored attacks.

It added: "While work is underway to develop those capabilities that will protect the UK's interests in cyberspace... (we are) concerned that much of this work remains preparatory and theoretical."