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Nearly one in six of British jihadists who have joined the ranks of Islamic State’s brutal regime have been killed, the Sunday Mirror can reveal.

Security bosses fear the number of radicalised fighters leaving the UK for IS strongholds in Iraq and Syria has rocketed to as many as 850.

Defence chiefs claim coalition air strikes and an increased military effort to halt the extremist group’s campaign of terror have wiped out at least 15% of all Brit fighters.

However experts claimed the Government had failed to eradicate ISIS and said the hundreds of fighters returning to the UK were a threat to national security.

Ministers have confirmed nearly half of the brainwashed recruits had been allowed back into the UK despite their links to the terror group.

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Col Richard Kemp, the former commander of all British forces in Afghanistan, said: “My concern is more with the 400 or so who have returned to the UK and pose a threat.

“We have seen what the Islamic State can do in Brussels, in Paris, in the US and most recently in Turkey.

“They can, and will, try to do the same thing here. I think that’s what is most concerning.

"We are not actually being effective in killing them in large numbers as we need to be.

“Our failure to wholeheartedly attack Islamic State gives people the inspiration to carry out attacks.

(Image: REUTERS/SITE Intel Group)

“Even if they are not directed by ISIS, many people around the world are inspired by the success of ISIS to carry out attacks and that’s a huge problem.

“The only way to stop ISIS from encouraging and inspiring people to attack us is to show how weak they are by destroying them and attacking them with great strength.”

RAF air strikes have killed hundreds of ISIS fighters in a wave of attacks across key positions in northern Iraq and eastern Syria .

Footage from Typhoon and Tornado jets show targets have been pounded with guided missiles while Reaper drones have taken out fighters with Hellfire missiles.

Mohammed Emwazi - the ISIS butcher known as Jihadi John - is among the highest profile Brits to be killed.

(Image: Twitter)

In November last year US counter terrorism officials revealed a “flawless” drone missile had “evaporated” the extremist as he stepped into a car in ISIS’s de facto capital of Raqqa, Syria.

Colonel Steven Warren said Emwazi, 27, from London, who beheaded several Western hostages on camera in ISIS propaganda videos, was “a human animal and killing him is probably making the world a little bit better place”.

In response to a parliamentary question, security minister John Hayes, said: “Approximately 850 UK linked individuals of national security concern have travelled to engage with the Syrian conflict.

“We estimate that just under half of those have returned and approximately 15% are deceased.”

Reyaad Khan, 21, from Cardiff, who also went under the named Abu Dujana al Hindi, was killed in the UK’s first drone strike in Syria last year on August 21.

Computer hacker, Abu Hussain al Britani, 21, who was convicted for illegally accessing the address book of former Prime Minister Tony Blair , was killed in a drone strike in August 2015.

Hussain went to Syria in early 2013 while on bail pending an investigation into an allegation of violent disorder.

But not all those who have died on foreign soil have been killed by coalition strikes.

In March this year a Brit calling himself Abu Musa al Britani was killed driving a car bomb into a convoy of army vehicles.

The jihadist was later exposed as being Mohammed Rizwan Awan, a 27-year-old British gas worker from Huddersfield in West Yorkshire.

(Image: Getty)

Col Kemp said: “Those returning from IS, those that can’t be arrested, have to be monitored.

“But you can’t monitor everyone - taking in to account the number of people affiliated to radicalised groups in the UK and those returning from Syria, it’s in the thousands.

“There will be a very small fraction of those suspected of links to IS will be monitored because the security services don’t have the capacity to monitor so many.

“The rest of them, we won’t know what they’re doing.”