A woman arrested at Heathrow with her baby is believed to be the first to return to the UK with a child born under Islamic State in Syria.
The woman, 37, whose identity has not been revealed, arrived on a flight from Addis Ababa and was arrested on suspicion of preparing terrorist acts under section 5 of the Terrorist Act 2006.
It is thought this is the first case of its kind involving a mother and a child born into the Isis regime.
MI5 has previously said around 850 extremists from Britain travelled to the Middle East as Isis took control of pockets of Syria and Iraq.
But as the terror group is pushed out of its former strongholds in Syria and Iraq, many of its militants with British passports are returning home with their families.
The woman who was arrested on January 12 is the latest in a string of arrests made by British counter-terror police bracing itself for an influx of jihadi brides.
Her child, who is reportedly under two, has been taken into care, according to the Sunday Times.
It is understood the woman flew from Ethiopia in an attempt to cover her tracks, but counter-terror officers were waiting at the airport for her when she landed.
A Metropolitan Police spokesman said: ‘She was detained under PACE and taken to a south London police station, where she currently remains in police custody.’
While the inquiry continues, the boy has been taken into care as counter-terror police continue to crack down on Britain-bound terrorists.
Last year, ministers stripped more than 150 jihadists of their citizenship and banned them from Britain, over fears they would return to carry out attacks in the UK.
In May, security sources said around 350 Isis fighters had already come back to the UK from Syria and estimate another 300 could yet return.
Among the blacklisted British citizens still thought to be living within Isis territory are Omar Hussain, a former Morrisons supermarket security guard-turned jihadi recruiter from High Wycombe, Aqsa Mahmood, a Glaswegian who went to Syria to marry an Isis fighter aged 20, and London-born Muslim convert Grace Dare, whose Isis husband Abu Bakr has since been killed.
The British ex-wife of a top US Isis member last week shocked Good Morning Britain viewers when she admitted she still loves her jihadist former husband.
Tania Joya, who became radicalised while growing up in Harrow, London, but now lives in Dallas, Texas, told Piers Morgan ‘everybody has their ups and downs’.
British anti-terror police have stepped up arrests in the wake of the Manchester, Westminster and London Bridge terror attacks last year.