Canada said on Sunday it was disappointed the United Kingdom had decided to "off-load their responsibilities" after its close ally revoked the citizenship of an Isil fighter known as Jihadi Jack.
Muslim convert Jack Letts, 24, who had held dual UK and Canadian citizenship, declared he was an "enemy of Britain" after travelling from Oxfordshire to Syria at the age of 18 to join the Islamic State of Iraq and the Levant.
He has begged to be allowed to return to the UK from Syria, insisting he had "no intention" of killing Britons, after he was captured by Kurdish forces in 2017.
The Home Office has now stripped Letts of British citizenship, meaning he is the responsibility of the Canadian government, The Mail on Sunday said.
Canadian Public Safety Minister Ralph Goodale confirmed in a statement that Letts had been stripped of his British citizenship.
"Canada is disappointed that the United Kingdom has taken this unilateral action to off-load their responsibilities," Goodale said.
A statement from the Home Office said revoking British citizenship was one way it counters terrorist threats. It said it does not comment on individual cases.
"Decisions on depriving a dual national of citizenship are based on substantial advice from officials, lawyers and the intelligence agencies and all available information," the statement said.
Letts has been behind bars in a Kurdish prison since 2017.
In 2015, Letts posted on Facebook that he would like to perform a "martyrdom operation" on a group of British soldiers. But by the end of that year, he was indicating that he wanted to come home.
The man’s parents are from Oxford and were found guilty earlier this year of sending him money. They received a suspended sentence.
Organic farmer John Letts, 58, and ex-Oxfam fundraiser Sally Lane, 56, said they were acting as any parents would have when they tried to send their son cash after he begged for help. They say they believed the 23-year-old was in mortal danger and trapped in Raqqa, Syria.
Police warned the family that "sending money to Jack is the same as sending money to ISIS."
Mr Goodale said terrorism knows no borders, so countries need to work together to keep each other safe. He also said Canada was under no legal obligation to bring Canadians detained in Syria home, and the government would not expose consular officials to undue risk.
Letts, who travelled to the Middle East in 2014, is now among more than 120 dual nationals who have been stripped of their British citizenship since 2016, including Isis bride Shamima Begum.
Ms Begum was one of three girls from Bethnal Green, east London, who left the UK aged just 15 in February 2015 and travelled to Syria to join Isil. It was thought Ms Begum may have a claim in Bangladesh because of her family background, something Bangladeshi officials denied.
The move can only be made against people with two passports, because international law prevents the Government from making anyone "stateless".