Shamima Begum could be arrested if she returns to the UK but many people coming back from Syria have led “peaceful lives”, Britain's most senior police officer has said.

Metropolitan Police commissioner Cressida Dick told journalists police would intercept the 19-year-old mother, who joined Isis with two friends from Bethnal Green Academy in 2015, on arrival.

“If she does, under whatever circumstances, arrive at our borders somebody in her type of circumstances could expect to be spoken to and if there is the appropriate necessity, to be potentially arrested and certainly investigated,” she added.

“If that results in sufficient evidence for a prosecution then it will result in sufficient evidence for a prosecution.”

Ms Dick said that if Ms Begum cannot be prosecuted, “we will assess the risk that she poses”, adding: “We do that with every single person who comes back from Syria and then manage the risk with colleagues in the [security and intelligence] agencies.“

Syria at war: Fleeing the caliphate Show all 14 1 /14 Syria at war: Fleeing the caliphate Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria Richard Hall/The Independent Syria at war: Fleeing the caliphate Zikia Ibrahim, 28, with her two-year-old son and 8-month-old daughter, after fleeing the Isis caliphate Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Men who fled the last Isis-held area of Syria line up to be questioned by American and Kurdish intelligence officials Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate A young girl pulls her belongings after arriving Richard Hall/The Independent Syria at war: Fleeing the caliphate An SDF fighter hands out bread to women and children after they arrive Richard Hall/The Independent Syria at war: Fleeing the caliphate Sita Ghazzar, 70, after fleeing from the last Isis-held territory in Syria Richard Hall/The Independent Syria at war: Fleeing the caliphate A family from Russia who recently fled the last Isis-held area of Syria Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria. Richard Hall Richard Hall/The Independent Syria at war: Fleeing the caliphate Richard Hall Richard Hall/The Independent

She acknowledged that travelling to Syria was not in itself an offence, and police would need evidence of a crime or terrorism for a charge.

“Some people returned from that area in the early days [of the war] who had almost certainly done nothing other than humanitarian aid work – we talked to them and assessed their risk,” the commissioner said.

“Many people have come back and just gone on with peaceful lives.”

In 2015, then head of UK counterterror policing Mark Rowley told MPs there was no evidence the Bethnal Green girls had committed terror offences.

“They have no reason to fear, if nothing else comes to light, that we will be treating them as terrorists,” he said, weeks after their disappearance.

Ms Dick said the comments were made “on the basis of what they knew then”.

Ms Begum has told journalists that she has “no regrets” over joining Isis, saying she was not fazed by seeing a severed head in a bin and describing the Manchester Arena bombing as “justified”.

She married a Dutch foreign fighter and after two of their infant children died, is appealing to return to the UK with her newborn son.

Amira Abase, Kadiza Sultana and Shamima Begum before catching flight to Turkey in 2015 (Met Police/PA)

The Independent understands that at least 20 British women and their children are currently in camps controlled by the Syrian Democratic Forces (SDF), as well as six male fighters from the UK.

Security services say that more than 400 of the 900 people of “national security concern” who travelled to Iraq and Syria have already returned and 20 per cent have been killed.

It is unclear whether those still alive will seek to return to Britain or travel to Isis strongholds elsewhere, such as in parts of Egypt or Afghanistan.

“What we don't know is how many people who are still out there will end up coming back,” Ms Dick said.

“We stopped people [travelling to Isis territories] wherever we could.”

Kurdish officials have urged the government to fulfil its “legal and moral duty” to repatriate British citizens for trial, but the home secretary has suggested that he will seek to block Ms Begum’s return.

It remains unclear whether any legal move could stop the British citizen re-entering the UK, but Syrian authorities say she will not be allowed to leave the al-Hawl camp without official assistance, and the Foreign Office has refused to give it.

Yesterday, Theresa May's official spokesman said: “Foreign fighters should be brought to justice in accordance with due legal process in the most appropriate jurisdiction.

“Where possible, this should be in the region where the crimes had been committed.