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SAS commanders have cancelled all leave as the regiment’s soldiers join police in the largest counter-terror surveillance operation in recent years. It comes as Home Office officials respond to the latest intelligence warning from the Joint Terrorism Analysis Centre, part of MI5, of a major attack by Islamic State extremists on mainland Britain. While the UK is already on a state of alert sources say they are responding to “new and urgent” threats which have emerged from captured ISIS militants in Iraq.

GETTY 200 undercover SAS agents will guard the British public from the threat of terrorism

And the undercover officers will be on high alert after the Berlin Christmas Market attack where a lorry ploughed through crowds. The Special Forces soldiers will operate under police direction but will be armed and ready to provide advice on hostage situations and any chemical incidents.

The risks at stake are profound and represent a fundamental threat to our sovereignty Alex Younger, MI6 chief

The revelation comes after a warning from MI6 chief Alex Younger about the “unprecedented” scale of the terrorist threat to mainland UK. He said: “The risks at stake are profound and represent a fundamental threat to our sovereignty. “They should be a concern to all those who share democratic values.”

GETTY Britain's secret intelligence services warn the likelihood of an attack is great

He also warned of an increased risk of “hybrid” threats, like cyber attacks. Intelligence and security services have disrupted at least 12 terror plots since 2013, many plotted in Iraq and Syria. According to MI5 chief Andrew Parker, responsible for security of the UK mainland, there may be as many as 3,000 home-grown Islamic extremists in the UK.

GETTY The SAS troops will go undercover and operate in small teams

Intelligence sources said Mr Younger’s warning signified that British security forces were aware of one or more terrorist cells actively on the move in the UK. The risk of a systematic attack on Britain’s communications infrastructure, designed to accompany a street level attack, was “very real”, the source added. The force of 200 soldiers, including specialists from a “human intelligence unit”, will join teams from the Police’s Counter Terrorist Command. Initially they will focus on major metropolitan areas such as Manchester, Birmingham and London, where police forces have requested additional support.

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GETTY The focus will be placed on bigger cities like London, Manchester, and Birmingham

Other Special Forces soldiers will operate in smaller teams to mingle with crowds of Christmas shoppers. For the past six months armed police officers have joined a special helicopter-borne SAS response force, which has been on standby to respond to “incidents” anywhere in the UK. A source said: “We have seen the military supporting the police before, but not on this scale.

GETTY The possibility of a cyber attack is also a major concern for security forces