The so-called Islamic State groomed and recruited hundreds of women and girls, who were forced into marriage, sexual slavery, domestic servitude and other forms of exploitation over several years.
Many British victims - including the east London schoolgirl Shamima Begum - have been left detained indefinitely in Kurdish-run refugee camps in north-east Syria and are at high risk of torture and illness.
An investigation by the charity Reprieve found that of the 800 Britons who travelled to Isis territory in Syria and Iraq, only 25 adults and 34 children are still in the region.
It is believed 84 per cent of them are women and children, while almost half (44 per cent) say they were coerced into travelling to the region by a male partner of relative.
Among those in a camp is a girl, known as Nadia, who was taken to Syria by a male relative when she was just 12. She was forced into marriage and repeatedly raped until she became pregnant at 15.
Nadia and her young son are now being held in camps with no prospect of ever returning home to the UK.
Maya Foa, Reprieve’s director, has visited the camps and said: “These are victims who have been abandoned. I find it cruel, callous and cynical.
“It is not lawful, not humane and not in our security interests. On every level it is counterproductive.”
Nadia is one of many girls and young women groomed by the Islamic State to join the terrorist group at the height of its power in the region.
Earlier this year the Supreme Court ruled Shamima Begum would not be allowed to return to the UK to pursue an appeal against the removal of her British citizenship.
Ms Begum, now 21, left her family home in Bethnal Green aged just 15 to travel to Syria with two school friends, Amira Abase and Kadiza Sultana.
She gave birth to a son, Jerah, who was a British citizen, but no efforts were made by the UK government to rescue him from the camp. He died after less than three weeks.
The then Home Secretary Sajid Javid responded by stripping her British citizenship, falsely arguing she would not be left stateless because her parents are of Bangladeshi origin.
Writing in the report’s foreword, Conservative MP Andrew Mitchell said: “There is no decency or justice in abandoning trafficking victims to face torture and the death penalty.
“These are difficult cases but Britain, as a leading member of the United Nations, must set a strong example. We cannot wash our hands of these Britons, abandoning them in ungoverned space.”
He said it is “the right thing to do” to bring British nationals back and added: “We must bring back all British nationals and tackle head-on the far-reaching ramifications of systematic trafficking by the so-called Islamic State.”
The Home Office is known to have made “citizenship deprivation orders” in respect of 19 of the 25 UK adults still in north-east Syria but Reprieve believes the number could be higher.
Siobhán Mullally, the UN’s special rapporteur on trafficking, said the repatriation of families currently detained in north-east Syria to the UK is a “necessary” step.
“The UK’s obligations, under domestic and international law, of prevention, of protection and of effective investigation of the crime of trafficking, must be fulfilled”, she said.
Ted Chaiban, Unicef’s director for the Middle East, previously pleaded: “We call on all member states to provide children - who are their citizens or born to their nationals - with civil documentation to prevent statelessness.
“This is in line with the best interests of the child and in compliance with international standards.”
Reprieve is calling for the UK government to follow the lead of other European nations - including France and Belgium - to repatriate British families stranded in north-east Syria and review their situations of a case-by-case basis.