Shamima Begum, who left the UK to join the Islamic State group in Syria aged 15, has a "right to return to Britain", Labour leader Jeremy Corbyn has said.
Ms Begum has had her UK citizenship revoked by Home Secretary Sajid Javid - a move Mr Corbyn said was "extreme".
The leader of the opposition told ITV News the 19-year-old should return to the UK to face questioning.
Ms Begum told Sky News on Thursday she was "willing to change" and called for "mercy" from British politicians.
UK nationals can only have their citizenship revoked if they are eligible for citizenship elsewhere.
It is thought Ms Begum could be a Bangladeshi citizen because her mother is believed to be one.
However, Bangladesh's ministry of foreign affairs has said Ms Begum is not a Bangladeshi citizen and there was "no question" of her being allowed into the country.
Mr Corbyn, who is currently in Brussels to discuss his Brexit proposals, said: "She obviously has, in my view, a right to return to Britain.
"On that return she must obviously face a lot of questions about everything she has done and at that point any action may or may not be taken.
"But I think the idea of stripping somebody of their citizenship when they were born in Britain is a very extreme manoeuvre indeed.
"Indeed, I questioned the right of the home secretary to have these powers when the original law was brought in by Theresa May when she was home secretary."
Mr Javid has defended the move, which followed a debate over whether the teenager should be able to return to the UK after she was found in a Syrian refugee camp.
Ms Begum, who left east London in 2015, said she never sought to be an IS "poster girl" and now simply wished to raise her child quietly in the UK.
The home secretary said he would not leave an individual stateless, which is illegal under international law.
But the Begum family's lawyer Tasnime Akunjee, who is preparing an appeal, has said he is considering whether she has been left stateless.
Ms Begum gave birth to a boy in a Syrian refugee camp at the weekend, who the home secretary has suggested could still be British, despite the removal of Ms Begum's citizenship.
"Children should not suffer. So, if a parent does lose their British citizenship, it does not affect the rights of their child," he told the Commons.
Mr Akunjee told the Guardian he planned to travel to the Syrian refugee camp "as soon as possible" to ask for Ms Begum's consent to bring her newborn son back to Britain, while her legal case is resolved.
"We can't do anything against her will, so I would hope that I would be able to outline the options for her, explain things to her," he said.
But Ms Begum told Sky News her son was unwell and she would not allow him to travel to the UK without her.
Ms Begum has previously said she had two children who both died.