The elite Special Forces have joined counter-terror police and 1,900 Army personnel in the biggest security operation since the 2012 London Olympics.

The news comes as Al Qaeda warned that France was only its third preferred target after Britain and the US.

Most of the Special Forces will be wearing civilian clothes, while some have donned police uniforms to accompany police officers who visit the homes of persons of interest in response to intelligence leads by MI5.

The security operation was rolled out after the Paris atrocities, in which 17 people, including three police officers, were killed as Islamic gunmen ran amok across the French capital over three days.

The bloodshed, masterminded by Al Qaeda in the Arabian Peninsula, sent shockwaves around the world.

On Wednesday, fanatical brothers Said and Cherif Kouachi ambushed the offices of satirical magazine Charlie Hebdo, known around the world for its regular depictions of the prophet Muhammad.

In the slaughter, 12 people were killed including five cartoonists, two police officers and the editor-in-chief Stephane Charbonnier.

In a chilling threat laid bare on news website The Intercept, an Al Qaeda spokesman said: “Why is France so thick in learning from its past mistakes. You come third in the target list, after US and Britain. If I were the latter, I would rather pull my sleeves up.”

The Sunday Express has learned that a 30-strong SAS team, divided into smaller groups, has been allocated to the Police Counter Intelligence Unit by the regiment’s inner sanctum, dubbed the Kremlin.

They form part of a “liaison plan” in which SAS troops work with the police in order to make sure that if required they already have good contacts.

An entire squadron of SAS soldiers is on permanent standby to react to an incident in the UK. Helicopters attached to 7 Squadron RAF were last night primed to rapidly transport the elite force.

A woman lays tributes at the scene of the Kosher market shooting in ParisAP

A woman lays tributes at the scene of the Kosher market shooting in Paris

Why is France so thick in learning from its past mistakes. You come third in the target list, after US and Britain

Al Qaeda spokesman

Separately, members of a covert “human intelligence” unit based in the Midlands are working alongside police and intelligence officers to identify threats.The “Humint” unit is thought to consist largely of Asian and Muslim operatives, and was used extensively in Afghanistan.

In addition members of the Special Reconnaissance Regiment were last night engaged in intercepting mobile communications.

A further 120 commandos of the Special Boat Service have also been called on to provide “maritime counter measures” to the Home Office.

Their task is to provide an immediate reaction force against the threat of a tanker or major vessel being hijacked off Britain’s coastline.

Regular exercises are held across the country and ports where local authorities, the police, military and Home Office test their procedures.

Last night senior sources confirmed that an additional 1,900 regular soldiers would be on standby as soon as they are selected from high-readiness regiments by Army HQ in Andover, Hampshire.

They will guard key installations with specialist intelligence units supporting the Home Office along with surveillance and electronic interception operations.

MI5 chief Andrew Parker warned that Al Qaeda radicals working out of Syria were planning mass casualty attacks in Britain and the West, targeting iconic landmarks.

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