SIR Keir Starmer insisted “national security” will be the priority when deciding if individuals such as jihadi bride Shamima Begum can return from Syria.
The Prime Minister was probed over his approach to dealing with stateless former Brits, saying cases would be handled individually.
Begum, 25, left her East London home aged just 15 to join IS.
But her future remains uncertain after the Assad dynasty was overthrown.
The PM told MPs: “I think the principle that we have to apply is that our national security comes first.
"So, that’s the way in which we will look at these cases.
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“Each of them will be different and I won’t comment on individual cases, but I do think the principle has to be our national security first.
“And that will be the primary consideration in the decisions that fall to be taken if and when they do fall to be taken.”
Downing Street has previously insisted there are “no plans” to allow her back into the UK.
Home Office Minister Dame Angela Eagle said earlier this week: “Her British citizenship has been stripped from her and that decision made by a previous government has been approved of at the highest levels in the courts, and that's the current situation
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The Minister added: “I'm not going to talk about her particular case.
“Every case would have to be looked on at … on its merits.
"And we would also have to have risk assessment about any such cases.”