Around 350 ISIS fighters have returned to the UK from Syria, creating a nightmare for security services, it emerged today.

MI5 has previously said around 850 extremists from Britain travelled to the Middle East as the terror group took control of Syria and parts of Iraq.

But with ISIS now being pushed out, jihadists with British passports are fleeing back to the UK, where authorities fear they may unleash a new wave of attacks.

Hundreds of British ISIS fighters have returned to the UK and pose a terror threat. File photo

Police and security services are said to be stretched to the limit trying to monitor extremists

Many of those returning were trained in the use of explosives and firearms as they took part in ISIS's bloody rampage,The Times reports today.

The paper states that intelligence gathered on them is often covert or sensitive meaning it cannot be used in court.

The flood of new jihadists is stretching the UK's security security services to breaking point, with up to 30 officers required to provide 24-hour monitoring of just one suspect.

It comes after a British defector from the group told how up to 300 British members of ISIS remain in the Syrian warzone.

Experts believe they may be in communication with those who have returned, raising the fear of further plots.

Tarik Hassane, nicknamed 'The Surgeon', plotted to kill UK police officers are returning to London following a trip to Syria and Sudan in 2013

Olivier Guitta, managing director at risk consultants GlobalStrat, told the Daily Telegraph this week: 'Many who joined Isil have returned to the UK and they could be awaiting instructions. It is very likely that there is communication.'

MI5 have previously said around 3,000 violent Islamist extremists are operating in the UK.

Erol Incedal was found bomb-making manual after he came back to Britain from Syria

British anti-terror police have stepped up arrests since Muslim convert Khalid Masood killed four pedestrians on Westminster Bridge before stabbing a police officer to death outside Parliament in March.

Masood is understood to have travelled to Saudi Arabia before the attack, but it is not believed he ever visited any ISIS-held territory.

A series of other terror plots whose perpetrators have travelled to the warzone have been thwarted, including that of so-called 'Surgeon' Tarik Hassane, who wanted to attack police.

ISIS is currently facing multiple offensives in both Iraq and Syria, with US-led airstrikes, Russian and Syrian forces as well as Kurdish and other groups pushing them back.

Iraqi forces said on Tuesday that they had retaken full control one of the city of Mosul's largest neighbourhoods.

Syrian forces have meanwhile entered Raqqa, the de facto capital of ISIS's so-called 'caliphate'.