Al Qaeda is plotting to massacre huge numbers of civilians in Britain and other western countries, the head of Mi5 has warned.

Andrew Parker, director general of the security service, told a select group mostly from the Royal United Services Institute at his London headquarters that while the so-called Islamic State is the newest threat to the West and that self-started lone wolf plots are hard to foil, al Qaeda still has ambitions for something more spectacular.

He said: "We know that terrorists based in Syria harbour... ambitions towards the UK - trying to direct attacks against our country, and exhorting extremists here to act independently."

But this was in addition to newly identified plans from al Qaeda to revisit their traditional style of operations.

He said: "…we still face more complex and ambitious plots that follow the now sadly well-established approach of al Qaeda and its imitators: attempts to cause large scale loss of life, often by attacking transport systems or iconic targets.

"We know, for example, that a group of core al Qaeda terrorists in Syria is planning mass casualty attacks against the West."

Al Qaeda experts are known to have moved from Pakistan to Syria, where they may be able to draw on British volunteers to conduct attacks in the UK. Their mission is entirely focussed on plots against the West.

They are known to be led by Muhsin al Fadhli, an expert bomb maker wanted by the FBI who has a $7m price on his head. He moved to Syria two years ago and was unsuccessfully targeted in a coalition air strike last year.

It is also known that British citizens are in the ranks of al Qaeda in Syria and could be used in a major atrocity back home.

The domestic spy chief spoke hours after the UK's COBRA committee ordered a boost to national security.

Home Secretary Theresa May said: "Following the attacks we took the precautionary step yesterday of increasing security at the French-UK border.

"The UK threat level, which is set by the independent Joint Terrorism Advisory Centre, remains at severe; that means a terrorist attack is highly likely and the public should remain vigilant.

"This morning I chaired COBRA to consider Britain's response to the attacks and our own preparedness to a similar attack."

Extra police have already been deployed at British ports and stations serving the Eurostar.

Lone wolf plots aimed at the police and military have surged over the last year and these have combined with the Paris killings to cause unease among officers.

"It's a huge worry for us," said Sir Peter Fahy, chief constable of Greater Manchester Police.

"There is a particular increased threat to police officers at the moment and there have been a number of attacks foiled in differed parts of the country, but I can tell you that our officers are very concerned about it… they see what happened in France, and clearly we are an unarmed police force but it is the nature of that threat which is worrying but despite that our officers go out there and do their duty."

Such concerns from front line police reflect similar fears in the intelligence community. Put simply - the world is getting more dangerous.