UK Supreme Court is to rule later whether the runaway schoolgirl Shamima Begum should be allowed back into the UK from Syria.
The ruling comes six years after the then 15-year-old left with friends to join the Islamic State group, reports the BBC.
The government stripped her of British nationality so she cannot return.
Five justices must decide whether the only fair way she can challenge that decision is to allow her back to take part in the case.
Begum's lawyers also argue she should automatically win her case to remain British, if she cannot receive a fair hearing.
The 21-year-old is in a camp controlled by armed guards in northern Syria.
Two years ago, the then-Home Secretary Sajid Javid stripped Ms Begum of her citizenship, following the collapse of the IS regime.
The Court of Appeal ruled last July Ms Begum had been denied a fair opportunity to challenge that decision because she could not make her case from the camp.
The government then asked the Supreme Court to reconsider the Court of Appeal's ruling - so Friday's outcome has potentially major implications for Ms Begum's case and others like it.