Former prime minister Helen Clark has weighed in on the debate over a British teenager who ran away to join the Islamic State group - saying she has a right to come home.

When Shamima Begum fled Britain with two other schoolgirls in 2015 to join the Islamic State, it shocked a nation. Heavily pregnant, she announced she wanted to come home and has now given birth to a baby boy.

Helen Clark told Newstalk ZB's Heather du Plessis-Allan that Shamima had made a terrible decision after being "groomed" by Isis - and that she should be allowed back into the United Kingdom.

"This girl, now a young woman, made the worst decision of her life when she was 15 and she made that decision when she was groomed by the Islamic State. She was recruited, she was a child," Clark said.

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"She is a UK citizen, she's born there. The head of MI6 says she has a right to return."

However, Clark says she should still face the full consequences of the law.

"She has a right to go home but they [MI6], of course, also have a right to investigate and follow through on whether she has broken British law."

When asked about concerns that Shamima might radicalise other British citizens, Clark said while it's always possible, she will be watched very closely.

"She is now under watch for the rest of her life. Security Services know exactly who she is, they will know where she is and if she doesn't end up in jail she will be under very close watch."

Clark said people could turn their lives around.

"It's always possible that people can turn their lives around. We have seen that before, we have seen people come to terms with what they did was deeply, deeply wrong and make amends."

Clark said her baby needs to be taken into account as well.

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She said the baby has been "born innocent" and needs to be considered and given a chance.

Clark said if she doesn't end up in jail, she should be put through a de-radicalisation programme.

"From what we are seeing, with the full support of her family who are aghast and humiliated by what has happened, so this is also the course taken with other returnees."

The former prime minister said her behaviour was abhorrent but she still deserves a second chance.

She said she understands that this case upsets people but says she is still a citizen.

"I'm appalled by the no regrets attitude that she showed, but nonetheless she is a citizen and in the end we do have an obligation."

"She did behave extremely badly but I come back to the point that she was a child and she was groomed."

"The Islamic State recruiters are very sophisticated and she is one of literally thousands of children and young people who have fallen for that grooming.

"These children were groomed by sophisticated terrorists who know which buttons to push and terrorists who would spend hours online with them."

Meanwhile a counter-terrorism expert says the law may be in the teenager's favour.

Will Geddes told Mike Hosking Shamima is still a British citizen.

"This particular individual is in a position where she is entitled potentially to return to the UK, however, there will be many conditions around that."

However, he said the British government will be worried about opening the floodgates.

"About 800 Jihadi fighters that they have imprisoned over in Iraq and Syria are potentially going to be released and come back, or try to come back to the United Kingdom."