The taxpayer could be forced to foot a £10 million security bill if Shamima Begum is allowed to return to the UK, it has been claimed.
On Thursday, the Court of Appeal ruled Begum, who travelled to Syria to join ISIS - will be allowed back into the UK to mount a "fair and effective appeal" against the decision to strip her of British citizenship.
According to The Sun, the ISIS bride would need to be monitored round the clock by police officers and security guards while awaiting trial.
The cost of accomodating her could reach £70,000, and there would be extra costs to pay if Begum is put on a Terrorism Prevention and Investigation Measure.
Dr Alan Mendoza, of the Henry Jackson Society think-tank, claims the bill could reach £10 million over the 20-year-old’s lifetime.
“It beggars belief that the taxpayer will likely have to fund a lifetime bill for Shamima Begum,” Dr Mendoza said.
“This money, that could reach £10million, would be better spent on the real victims of terrorism in the UK.”
It comes as Boris Johnson said on Sunday that ministers are looking at changing the legal aid eligibility rules after the Court of Appeal ruling.
Mr Johnson told The Sunday Telegraph: "It seems to me to be at least odd and perverse that somebody can be entitled to legal aid when they are not only outside the country, but have had their citizenship deprived for the protection of national security.
"That, amongst other things, we will be looking at."
The Prime Minister said that the Government would also be looking at the whole system of judicial review to establish whether it had "perverse consequences".
"What we are looking at is whether there are some ways in which judicial review does indeed go too far or does indeed have perverse consequences that were not perhaps envisaged when the tradition of judicial review began," he said.
Begum, now 20, travelled to Syria in February 2015 and lived under IS rule for more than three years before she was found in a Syrian refugee camp in February last year.
Then-home secretary Sajid Javid revoked her British citizenship on national security grounds later that month.
Begum took legal action against the Home Office, claiming the decision was unlawful because it rendered her stateless and exposed her to a real risk of death or inhuman and degrading treatment.
The Home Office spokeswoman said it would be applying for permission to appeal against the court's judgment.