Ex-British Army veteran Alan Duncan, 50, spent two years volunteering as a mercenary for the Kurdish Peshmerga in the fight to drive ISIS out of Iraq.
Moved by images of children being orphaned by ISIS, Mr Duncan, who fought for Britain in the Gulf War, joined the conflict in 2015.
Two years on and ISIS has lost all its major territories in Iraq after government forces reclaimed the city of Mosul – where the caliphate was established in 2014.
But despite Iraq forces declaring victory over ISIS on the battlefield, Mr Duncan told Daily Star Online the nature of the jihadist threat has evolved.
Mr Duncan, who is now back home in Scotland, believes British jihadists who fought in Iraq will flock back to the UK now ISIS is finished.
His stark comments come after a cell of depraved ISIS-linked terrorists killed 14 people and injured around 130 in two horrific attacks in Spain.
The van who ploughed into the crowd, killing at least 13 people and injuring around 100 others is towed away from Las Ramblas in Barcelona
Kurdish security forces warned the UK government about British Muslims traveling to Iraq to “get radicalised and trained” three years ago, Mr Duncan said.
Having been brainwashed by ISIS propaganda, Mr Duncan believes these fighters will now become recruiters for terror in the UK.
It is estimated that around 850 people from the UK have travelled to support or fight for jihadist groups in Syria and Iraq.
Around 350 ISIS fighters are thought to have returned to the UK, creating a nightmare for security services.
But despite his valiant volunteer work, Mr Duncan claims he has been subject to several episodes of “harassment” at the hands of British authorities.
“Forget Manchester and London, I think what we’ve seen so far is nothing.”
He said: “The Kurdish security were warning the UK and other western countries three years ago about western fighters coming across.
“They also said look, these guys are going to be coming across here to get radicalised and trained.
“Then they’re going to go back to your countries and slowly build up and form teams that recruit and radicalise.
“A lot of them may even switch over the al-Qaida once ISIS are defeated. That’s the threat that you’ve got over in the UK now.”
Britain has been rocked by three major ISIS-inspired terror attacks in 2017 with 39 deaths in all.
The first bloodbath took place in March on Westminster Bridge in London when 52-year-old attacker Khalid Masood mowed down pedestrians before stabbing a policeman.
The second saw Salman Abedi, 22, detonate a shrapnel bomb outside Manchester Arena just two months later – killing 22 people, mostly children.
A trio of attackers carried out the third knife and van attack on London Bridge in June, killing 11 revellers on a Saturday night.
In every case, the attackers were homegrown and are understood to have not been fighting with, or radicalised by, ISIS in the Middle East.
But Mr Duncan said ISIS losses on the battlefield will translate into more terror back home – from returning jihadists, homegrown or otherwise.
He said: “Forget Manchester and London, I think what we’ve seen so far is nothing.
“We’re going to see more of it, the more that ISIS lose.
“The UK security has done a brilliant job stopping attacks.
“But you can’t get them all. A lot of ISIS members are trying to sneak into the UK with refugees.”
Mr Duncan, formerly of the Queen’s Own Highlanders regiment, said the only way to deal with the problem is to deport British jihadists to face justice abroad.
The wreck of the Number 30 double-decker bus in Tavistock Square, one of four bombs set off by terrorists on July 7th 2005, which killed 52 and injured over 700 people
He said: “Any UK Daesh that’s returned has given up their rights. They have to be locked up, no ifs, no buts.
“You have to locked them up summarily and then deport them back to Iraq, Kurdistan and Syria.
“They have to stand trial across there.
“Why is it okay for them to commit mass murder, mass rape and mass child molestation, and then come back to this country as if nothing has happened.”
In what is believed to be a revenge attack against Muslims, a man drove a van into a group of worshippers outside a Mosque in Finsbury Park, London in June.
Mr Duncan said retaliation attacks only play into the hands of ISIS, whose ideology is rooted in hatred towards other religions, namely Christianity.
He said: “The other groups that you need to watch out for are on the far right.
“The Far Right is using the Manchester and London attacks for their own agenda.
“If there’s one thing that ISIS wants, it’s a holy war spurred by hate and division.”