A decorated army veteran renowned for rescuing youths from Islamic State today warned that the terrorist group is placing 'sleeper cells' in European countries including the UK.

Dimitri Bontinck, who is known as the 'Jihadi Hunter', spoke out as he prepared to support his own teenage son in a criminal trial after bringing him home from Syria.

It came as Home Secretary Theresa May and senior British police chiefs also said that a terrorist atrocity involving an ISIS-style beheading or bomb attack on civilians is now 'almost inevitable'.

Referring to their campaign of atrocities carried out abroad, Mr Bontinck, 41, told MailOnline: 'It's not the kids who are the principal danger – it's the chiefs whom you never see in the videos.

'The mature men running this organisation are deeply sinister, and extremely well-funded, and they have European countries such as Britain, France and Belgium in their sights.'

Belgian Army veteran Dimitri Bontinck (centre) poses with contacts in war-torn Syria. He has warned that the Islamic State is placing sleepers cells in the UK and other European countries after lisasing with members of Jabhat Al Nusra, a branch of Al Qaeda, and other contacts with close links to ISIS operatives

Some of the contacts Mr Bontinck met in the Syrian town of Al-Hamraa during his missions to find his son

He added: 'Their aim is to take revenge against the west for what they view as an all-out assault on them, and they will strike when they are ready.

'I've been told by very influential sources that they have sleeper cells over here, and are preparing to unleash their war in Europe.'

Mr Bontinck, a Belgian national who is based in Antwerp, has completed seven missions to Syria to help rescue his son, 19-year-old Jejoen, along with other youths in the same position.

The special forces trained negotiator insists that Jejoen is innocent of any wrongdoing, and that the authorities should not be 'stigmatising and criminalising youngsters who fall in with the wrong crowd'.

Jejoen was raised as a Catholic, attending a Jesuit school in Antwerp, but converted to Islam at 15 after going out with a local Muslim girl.

He became radicalised under the influence of the now-banned Sharia4Belgium group and then travelled to the Syrian civil war zone after flying to Turkey.

Dimitri Bontinck arrives in Turkey (left) in April 2013 on one of seven missions to find his son Jejoen, who he was reunited with in Syria earlier this year (right)

Mr Bontinck with his son’s passport – the one which he did not need to travel from Belgium to Syria

Jejoen told his parents – Mr Bontinck and his mother, Rose – that he was 'on holiday with some mates in Holland'.

'He didn't even have a passport to get to Turkey, instead using his Belgium identity card,' said Mr Bontinck, who had to travel to and from Syria three times before finding Jejoen.

'We were both accused of being spies at one stage, and were badly beaten up' said Mr Bontinck, adding: 'But the main thing is that we made excellent contacts with the radicals, as well as other influential players, before getting out.'

Mr Bontinck's main dealings were with Jabhat Al Nusra, a branch of Al Qaeda, but he has also spoken to those close to ISIS operatives.

His exact movements are shrouded in secrecy, but he has shown MailOnline documents and other evidence which confirm dealings with intelligence agencies and other governmental groups, as well as warring factions.

Pictures show Mr Bontinck armed and working with 'my team' out in Syria – a group which includes Arab speakers and those who 'know the country intimately'.

He says he has received death threats from ISIS, and believes his son is in 'serious danger', and should be given 'a new identity after offering the authorities a huge amount of information about those who indoctrinated him'.

A picture taken by Mr Bontinck during one of his tough negotiating sessions in war-torn Syria earlier this year. His exact movements are shrouded in secrecy, but he has shown MailOnline evidence which confirm dealings with intelligence agencies and other governmental groups as well as warring factions

Growing threat: Youngsters learning about radical Islam on the streets of Antwerp in Belgium

Jejoen, who was rescued earlier this year, was held for a time with James Foley, the US journalist who was later murdered, and the Briton John Cantlie, who continues to appear on IS propaganda videos.

'They all knew each other as the three Js', said Mr Bontinck, blaming both American and Britain for 'not working hard enough with the radicals to get their subjects home'.

Mr Bontinck has so far helped to bring four other youngsters, beyond Jejoen, back from Syria, and said there were 'too many intelligence agencies who have missed opportunities to rescue hostages.'

He helped in an attempt by Khadra Jama, from Manchester, to rescue her twin daughters Zahra and Salma, 16, who have married IS fighters since running away in June.

An older brother, Ahmed, was already involved in the conflict, having joined Al Shabab in their parents' native Somalia.

Mr Bontinck said a charity worker based in Denmark met one of the sisters near Aleppo after being promised safe passage by an IS commander, but the teenager was not persuaded to leave.

This was despite assurances that she could go to Denmark, where the family has connections and where the authorities adopt a more understanding approach to those who return.

Jejoen Bontinck with his barrister, Chris Luyckx, back in Antwerp. He and more than 40 others with alleged links to Sharia4Belgium are currently on trial accused of belonging to a terrorist organisation

On a second trip, to meet the second daughter, the charity worker also took the twins' mother. Both were captured and briefly held by IS before being freed, but without fulfilling their mission.

'It's extremely dangerous out there, and of course I fear for my life,' said Mr Bontinck. 'I work with some excellent, people, however. We are a team and we are committed to getting youngsters home.

'My greatest help has come from the French, who are very proactive in getting their people out. They do a lot of their dealings through Qatar.'

The massively wealthy Arab country has close links with France despite – controversially – being accused of indirectly funding IS as an enemy of Syrian president Bashar Al-Assad.

'The French are committed to rescuing hostages, in a way which Britain and America are not,' said Mr Bontinck.

Mr Bontinck said he 'had no evidence' of cash being used to free hostages, saying 'all I have used is negotiation with the right people.'

Proposals for improving the situation offered by Mr Bontinck include an international effort to close down Jihadi websites, and stricter passport controls on young people travelling to Turkey.

Jejoen Bontinck, who is suspected of being part of Sharia4Belgium, arrives for his trial in September

'These are just simple measures which would make the situation better straight away,' said Mr Bontinck.

He insisted 'Jejoen was a victim, just like so many other teenagers and other young people who get caught up in this type of madness.

'These radical Islamic groups are no different from cults. They create a sense of community among young people, and then lead them into terrifying situations.'

This week, Mr Bontinck will attend court for the end of a trial in which his son and more than 40 others with alleged links to Sharia4Belgium are accused of belonging to a terrorist organisation.

Jejoen has denied any wrongdoing, saying he went out to Syria for humanitarian reasons, and spent a great deal of time locked up.

He was also held in prison in Belgium for more than a month on his return, before being freed on stringent bail terms.

Prosecutors have demanded a four year jail term for Jejoen – something his father said was 'preposterous'.

A 15-year term has been sought against Fouad Belkacem, the Sharia4Belgium spokesman and alleged mastermind of the group.

ISIS executioner Jihadi John and his henchmen prepare to behead Syrian soldiers in the same sickening propaganda video that shows the decapitated body of American aid worker Peter Kassig

'We will chop off the heads of whoever you bring': A British ISIS fighter dares west to send ground troops in this video rant. Mr Bontinck has warned that ISIS will strike in the West when they are ready

Belkacem is notorious for posting Jihad recruitment videos online, even though Sharia4Belgium was officially dissolved two years ago.

Mr Bontinck, who was twice honoured when part of a UN peace keeping forced in the Balkans in the 1990s, said his son was 'victim, key witness, and terrorist suspect' in the trial.

The highly trained former solder is now in demand as a 'Jihadi Hunter' who will travel to some of the most dangerous places in the world to rescue youngsters.

Requests have come from numerous countries, including Britain, but he said he was finding it 'increasingly hard to be the Mother Teresa figure' without more support.

'The Belgian government has no interest at all in doing anything positive, and nor do countries like Britain, so a non-governmental agency is necessary, or at the very least charitable donations. The priority has to be to save our children.'

Hendrik van de Velde, the Belgian foreign affairs department spokesman said 'Belgium was the first country to draw attention to the problem' of young men and women being radicalised, adding: 'Belgium also seeks to avoid radicalisation within society through grassroots work at local level.'

Jejoen Bontinck has kept his Islamic faith, but has renounced Sharia4Belgium, and now gets on 'extremely well' with both Mr Bontinck and his Nigerian-born wife, Rose.

In speeches today, Theresa May and senior police will warn that the 'diverse' terrorist threat posed by jihadis returning from Syria and Iraq is one of the greatest Britain has ever faced.