The government is putting Britons at risk by depriving Isis members of their British citizenship, a former defence minister has said.
Tobias Ellwood told The Independent the detention of thousands of jihadis and their families in Syria was creating conditions for an Isis resurgence.
“We’ll see Daesh 2.0,” he warned. “We’ll see a repeat of al-Qaeda regrouping and becoming a very real threat, and that threat won’t just pose itself in the Middle East, but also to Britain.”
The former soldier, whose brother was killed in the 2002 Bali bombings, said more British victims had been killed in terror attacks abroad than in the UK.
“We’ve still got attacks taking place, we’ve still got the ideology alive,” said Mr Ellwood, who was sacked as defence minister by Boris Johnson.
“We’ve done well to stand up as a lead nation on the battlefield in defeating Daesh and the caliphate, but the last piece of the jigsaw is 20,000 or so fighters that nobody really wants.
“They will regroup to fight another day - we’re already seeing it.”
Mr Ellwood said the UK had not “concluded our mission” to defeat Isis, adding: “We need to complete it rather than allowing it to haunt us in the future.
“We stepped forward because we had a sense of duty, of values and standards, and if we just give up on that we’ve forgotten what we were fighting for.”
The move means the government will not repatriate them from Syria for trial, has cancelled their passports and would bar any attempted entry to the UK.
Bangladesh and Canada hit out at the government for “offloading Britain’s responsibilities” over Begum and Letts, who had publicly pleaded to return home.
They are among thousands of alleged Isis members detained by the Syrian Democratic Forces (SDF), whose pleas for the UK to take back its jihadis have so far been refused.
The Independent understands that dozens of British Isis members and at least 30 of their children are in custody.
As a foreign minister, Mr Ellwood attended meetings in 2015 and 2016 where international members of the US-led coalition drew up plans against Isis.
ISIS brides seen raising their index fingers in Stacey Dooley's Panorama documentary
“We talked about cutting off the financing, cutting off the online presence and recruitment, cutting off the connectivity and stopping the movement of people into Isis territory,” he recalled.
“The one thing we never did was then say ‘ok fine, when the caliphate finally falls and you get a mass exodus or surrender of people as you do in any fall of a regime, what is the process?’
“At the moment they’re just behind barbed wire with no process whatsoever and the SDF unable to manage it.”
Hundreds of Isis prisoners have already been released from jail in Syria after the Kurdish authorities holding them said they had “no blood on their hands”, and there are fears that more will be freed without any formal assessment of the danger they pose.
Abdel Karim Omar, a Kurdish foreign affairs official, told The Independent that thousands of detained Isis fighters, women and children are a “big burden”.
“They belong to 49 countries, and they don’t have documents and passports,” he said, speaking earlier this year.
“We cannot bear this responsibility alone. We ask the international community and the countries to which Isis members belong to take up its moral and legal duty and repatriate their citizens back to their countries.”
The British government has refused the call and has not yet proposed an alternative judicial process or long-term solution for the detainees.
Mr Ellwood said that in the early days of the battle against Isis, where the UK supported Kurdish forces on the ground with airstrikes, few prisoners of war emerged because fighters were either killed in battle or committed suicide attacks.
As Isis territory shrank, the US-led coalition vastly underestimated the number of people inside its remaining pockets of territory and did not prepare for the surrender of thousands of men, women and children in March.
Tens of thousands were crammed into the al-Hol refugee camp, where the black flag of Isis was raised in July.
Mr Ellwood warned that the detention centre was “haemorrhaging” jihadis, amid bribery allegations, while those still inside were free to radicalise each other.
The Conservative MP, who was hailed a hero for his efforts to save the police officer murdered in the Westminster attack, said a new strategy was needed to deal with committed extremists who are willing to die for their cause.
He called on international leaders attending the UN General Assembly next month to form a solution, which could include a designated legal process for Isis members.
Isis claimed responsibility for a suicide attack that killed 80 people at a wedding in Afghanistan last week, and its propaganda channels churn out daily reports of executions, shootings, battles and bombings across the Middle East, Asia and Africa.
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In the past two months, Isis has released more than a dozen videos showing factions in different countries renewing their pledges of allegiance to leader Abu Bakr al-Baghdadi.
“The afflictions and tribulations experienced by your brothers in [Syria] only made them more steadfast and patient,” said one fighter, following footage from Yemen, Bangladesh, Somalia, Tunisia, Libya, the Philippines, Turkey, Egypt the Caucasus and elsewhere.
A 2016 report commissioned by the government warned that removing extremists’ citizenship left them free to continue terrorist activities abroad, prevented monitoring and encouraged the “dangerous delusion that terrorism can be made into a foreign problem”.
But the use of the controversial power soared by more than 600 per cent in a year, and has been applied to more than 150 people since 2010.
A Home Office spokesperson said: “The home secretary has the power to deprive someone on conducive to the public good grounds of their British citizenship, where it would not render them stateless.
“Depriving someone of their British citizenship is never a decision that is taken lightly. Decisions are based on advice from officials, lawyers and the intelligence agencies and all available information to keep our country safe.
“Those who have fought for or supported Daesh should wherever possible face justice for their crimes in the most appropriate jurisdiction, which will often be in the region where their offences have been committed.”