Shamima Begum, who left Britain at the age of 15 in 2015 to join ISIS in Syria, will not be allowed to return to the UK.
British Home Secretary Priti Patel said she rejected Ms Begum’s request to return to the UK.
Ms Patel said the teenager, who married Dutch-born ISIS militant Yago Riedijk, was told there was “no way” she would be allowed to return after more than three years of living under the rule of the extremists.
"We don't need people who have done harm and left our country to be part of a death cult and to perpetrate that ideology," the Home Secretary told The Sun newspaper.
"We cannot have people who would do us harm allowed to enter our country, and that includes this woman.
"Everything I see in terms of security and intelligence, I am simply not willing to allow anybody who has been an active supporter or campaigner for [ISIS] in this country."
The teenager from east London made headlines when she left for Syria through Turkey with two friends.
Ms Begum drew public attention in recent months after she was found heavily pregnant in a refugee camp in Syria in February.
Since the death of a son she gave birth to in March, Ms Begum has repeatedly asked to return to the UK, despite a decision by Ms Patel’s predecessor, Sajid Javid, to strip her of citizenship.
Ms Begum has said that after the deaths of three children, she has suffered mental health problems and now deeply regrets joining ISIS.
Meanwhile, another ISIS member from the UK asked to be allowed to return.
Tooba Gondal, 25, left her home in London to join ISIS the same year as Ms Begum.
During her time in Syria, Ms Gondal was married three times to ISIS militants, all of whom died.
She gave birth to two children who survived the escape from ISIS’s diminishing territory.
Ms Gondal told The Sunday Times that other members of the militant group were "criminals" and she asked to be allowed to return home with her children.
