A British-born woman who went to Syria as a teenager to join ISIS will still pose a threat to national security if she is allowed to return, the UK's top court has heard. Shamima Begum, who was born to Bangladeshi parents, left London in 2015 when she was 15 and went to Syria via Turkey with two school friends. She was stripped of her British citizenship last year, but in July the Court of Appeal unanimously agreed that Begum, now 21, could only have a fair and effective appeal of that decision if she was permitted to come back to Britain. Now, the UK’s Home Office has taken the case to the Supreme Court to challenge that decision. On Monday, Sir James Eadie QC, for the British government, argued she should not be allowed to return. "The assessment was that she presented a current threat, justifying the removal of her British citizenship and thereby placing serious practical and legal impediments on national security grounds in the way of her return to the United Kingdom," he told the court. “If you allow it the effect is to create a very serious national security concern. “She was aligned with ISIS, she was radicalised when young and is just as much a threat as a person radicalised later. “There have been no allegations of trafficking or grooming made. Even if she was radicalised before departure it does not alter the threat. “It is plainly right that the public would be at an increased risk to safety on her return.” In Syria, Begum married an ISIS fighter and lived in Raqqa, where she remained for four years until she was discovered in a detention camp. She had three children after leaving Britain, but all the infants have since died. Sir James told the Supreme Court that removing citizenship was an “important” tool in the UK’s armoury to protect the nation. “Resources are not unlimited and it has been graphically illustrated by the London Bridge attack where the people concerned had been on the agency’s radar and had nevertheless been able to perpetrate the atrocity,” he said. “Those who have participated in attacks and attempted plots in the UK have historically come from a variety of sources and backgrounds. Like the 7/7 bombers coming out of a clear blue sky. “The exposure of the public to an increased risk of terrorism is not justifiable or appropriate in this case on fairness grounds.” Ms Begum’s legal team says allowing her to return to argue her case is the only fair path. “To allow the national security consideration to deny Ms Begum access to a meaningful right of appeal is to put the cart before the horse,” Lord Pannick QC told the court. “Leave to enter is the only such means available.” The Supreme Court is considering whether Ms Begum should be allowed to return to the UK to appeal against the deprivation of her British citizenship and whether, if she is refused leave to enter the UK, her citizenship should be restored. “The power to deprive a person of British citizenship is one of the most severe and intrusive powers that Parliament has conferred on the Secretary of State,” Lord Pannick QC said in written submissions. “A deprivation order is far more serious in its consequences than most criminal penalties.” The Supreme Court hearing is due to last two days with a decision expected to be handed down at a later date. Begum is currently in the Al Roj detention camp in northern Syria.