GETTY Met police officers train for a terrorist attack in London

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And experts are convinced a cell from this massive group of extremist Islamists are poised to launch a Paris-style attack in London, Birmingham, Manchester, Glasgow, or any number of vulnerable British cities. Around 750 UK-born extremists have fled to Syria to join ISIS savages fighting in Iraq and Syria.But around half of those are believed to have returned to these shores. The Home Office has disclosed to Express.co.uk that more than 375 Britons allied to the evil extremist group - also known as Daesh - are now back in the UK.

Far from being locked-up on their return they have been allowed to return to their pre-ISIS lives, with at least one taking up his university course again. In the wake of the Paris attacks last month, for which ISIS has claimed responsibility, security officials have warned that Britain is now a prime target. A leading terror analyst claimed today he had received a tip-off that "ISIS in Syria and Iraq is aiming to hit the United Kingdom next” - with returning fighters linked to the plot. Paul Cruickshank said there was “intelligence suggesting that British ISIS operatives in Syria and Iraq have been tasked to return to the United Kingdom to launch an attack against the UK. "These concerns have been compounded by that vote yesterday in the British House of Commons authorising British airstrikes for the first time against ISIS in Syria."

GETTY Armed police are a familiar site in the capital

The staggeringly high number of returning Islamist fighters is now putting the Britain’s police and intelligence services under unprecedented strain. Hannah Stuart, a terrorism expert at the Henry Jackson Society, said Britain’s security services were severely “overstretched”. She told Express.co.uk: “The high number of foreign fighters now back on British streets undoubtedly puts an increased strain on Britain’s intelligence services – and, with seven foiled terrorist plots in the last year, they are already overstretched." Worryingly, the Home Office has refused to reveal how many - if any - of the 375 fanatical Islamists are being monitored by MI5 or the police. According to Colonel Richard Kemp – former head of the international terrorism intelligence team at the Cabinet Office and ex-chairman of the Cobra crisis group - to monitor just one suspect 24 hours a day “you would probably needed 15 to 20 people – three shifts of three to five people on the ground plus at least three people on shifts back at HQ as well as support staff. “The intelligence services need more resources and more powers.”

Airstrikes against ISIS Sat, February 27, 2016 British, French, Russian and US forces launch air strikes around the world. Play slideshow 1 of 63 Russian army carried out an airstrike in Damascus

By that calculation, Britain would be home to 450 depraved attackers. And he said ISIS was also encouraging fanatics who cannot get to the Middle East to carry out attacks here. Roughly 70 of the British jihadis – who include London-raised Jihadi John who specialised in beheading Western hostages – have been killed. Jihadi John himself was killed in a drone attack earlier in November. Scotland Yard is investigating 600 terror cases relating to Syria and Iraq – a doubling in 18 months.

IG Islamic State militants

GETTY RAF jets are pounding ISIS in Syria and Iraq