Shamima Begum was legally a child when she left behind a comfortable life in Bethnal Green, London, to join the Isis caliphate in Syria. Now 19 years old, and pregnant, she wants to come home.

She is not the only British “Isis bride” being held in detention by Kurdish forces in Syria. About 20 British women and children, caught leaving the caliphate, are being held in camps by the Syrian Democratic Forces, along with six suspected fighters, The Independent understands.

The British government has said it won’t take them back. What happens to them now is the subject of a push-and-pull between the local Kurdish forces and the British government. They remain in a legal limbo, unwanted by their captors or their own country.

Is there a chance British fighters and families will return to the UK?

The British government has been seeking to block the return of anyone considered a security threat with a range of legal powers.

Many of the known Isis members who were dual nationals have had their British citizenship removed, like members of the “Beatles” cell, invalidating their travel documents and meaning the UK can avoid responsibility for repatriating them from Syria or future legal proceedings.

The most recent official statistics show that 104 people were deprived of British citizenship in 2017 and 14 in 2016.

The power has been used for criminals as well as terrorists, and the law allows it in cases “conducive to the public good”.

But making a person stateless is against international law, so it can only be used for dual nationals and in November two cases were ruled unlawful.

The government said the two alleged Islamists were dual British-Bangladeshi nationals, but the Special Immigration and Appeals Commission found they had been made stateless.

Passports can also be refused or withdrawn from a terror suspect, with the power used more than 80 times since it was introduced in 2013.

British citizens can be subjected to temporary exclusion orders (TEOs), which are imposed by the home secretary and make it illegal to return to the UK without notifying authorities.

Subjects are put on “no fly” lists, ensuring that their return is flagged, and they then have to abide by conditions such as reporting to police or entering deradicalisation programmes.

Nine TEOs were imposed in 2017 and any breach is a criminal offence.

Shamima Begum, 15, Kadiza Sultana, 16, America Abase, 15, fled to Syria in 2015 (EPA) (Metropolitan Police/EPA)

How many Britons joined Isis, and how many have returned so far?

According to a government estimate, at least 40 per cent of the 900 people of “national security concern” have returned and 20 per cent have been killed.

But the assessment is more than a year old and predates recent military advances that resulted in Isis territory being almost eradicated.

Separate research by The Soufan Centre in 2017 estimated that at least 425 British Isis members had returned to the UK – the largest cohort in Europe at that point.

Security services believe they know the identities of the vast majority of jihadis who joined the terrorist group and returned, and conduct risk assessments for those who cannot be prosecuted.

But it has been impossible to rule out the possibility that not all those who travelled to Isis territory were identified when they fled the UK.

Those that have avoided being put on a watchlist would not be identified on return to the UK, because their arrival would not be flagged to authorities.

In October, the head of UK counterterror policing, Neil Basu, told MPs: “The 40 per cent [of 900 recruits from the UK] who came back quickly are not my concern, my concern is the battle-hardened terrorists.

“I am satisfied that we have coverage on the people who we know travelled and returned … [but] assuming that we know everyone who travelled and everyone who came back is the difficulty.”

Where are they being detained?

Most “Isis families” are being held in al-Hol camp, in northern Syria. More than 35,000 people are now living in the tent city, which has become desperately overcrowded in the past month as thousands have fled the shrinking caliphate.

Local Isis family members are free to leave the camp if they have a sponsor and have passed the SDF checks. But westerners are detained in the camp.

The International Rescue Committee, which works inside the camp, is short of about 2,000 tents, “meaning 4,000 people are sleeping rough at the camp’s reception area exposed to rain and freezing temperatures”.

A total of 51 babies and children have died either on the way to the camp or shortly after arrival, due to malnutrition or lack of proper medical care.

The suspected fighters are held in secret jails in northern Syria by the SDF.

Are Britons who joined Isis liable for prosecution in the UK?

Yes. As Theresa May’s official spokesperson said on Thursday: “Any British citizen who does return from taking part in the conflict must be in no doubt they will be questioned, investigated and potentially prosecuted.

“Decisions on how people are dealt with are made on a case-by-case basis to ensure the most appropriate action is taken. Whatever the circumstances of an individual case, we have to, and will, protect the public.”

Security services have a watchlist of known Isis members and aim to intercept any returning via air or sea ports.

They are interviewed and the material is added to evidence and intelligence gathered during their time with Isis.

Authorities then decide whether they can be prosecuted for conventional offences, like murder, or terror offences such as preparing terrorist acts or membership of a proscribed group.

But the chaos in Syria and Iraq has made it almost impossible to prove people’s activities, and few returnees have been prosecuted.

A man who travelled to join Isis in 2014 and returned was jailed for seven years for preparing acts of terrorism.

The new Counter-Terrorism and Border Security Act, which received royal assent this week, contains an offence aimed at foreign fighters making it illegal to remain in a “designated area”.

But no areas have yet been designated, it cannot be applied retrospectively and people have to be there voluntarily for the law to work.

That means it cannot be applied to Isis members who have already returned to Britain, or anyone in custody in Syria.

What happens if returned Isis members cannot be charged?

They can be monitored and put under restrictions as part of terrorism prevention and investigation measures (TPIMs).

They can force people to move to another part of the country, exclude them from places, ban association with certain people, limit their phone and internet use and stop them leaving the country.

The extensive powers are used in only a small number of cases, with eight in force last year, because of the huge resources needed to enforce them.

Anyone not reaching the bar for a TPIM can be monitored in other ways by the security services, but Mr Basu recently warned that the system was running “red hot”.

There are a record 700 terror investigations running, with 3,000 high-risk extremists on an MI5 watchlist. Another 20,000 people have appeared on the security services’ radar in the past.

Can the families of Isis members face charges?

Any adults known to have lived in Isis’s self-declared caliphate will face criminal investigation whether they fought or not.

Relatives in the UK are dealt with on a case-by-case basis and can be prosecuted for committing any terror offences themselves.

Dozens of relatives have been jailed for “terrorist fundraising” after sending their loved ones money in Isis territories.

Those found to have known of plans may also be charged with “not disclosing the information as soon as reasonably practicable”.

If Isis members and their relatives have shared terrorist propaganda, they can also be charged under dissemination laws, and a new offence has been created making the viewing of terrorist material online punishable by up to 15 years in prison.

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

What will happen to the children of suspected Isis members?

The fate of children and teenagers who were born in Isis territories or taken there by their parents remains unclear, and there is fierce debate about whether they should be considered victims or a security threat.

Isis has publicised its use of “cubs of the caliphate” camps to indoctrinate young boys and train them as child soldiers.

Young children, including the four-year-old son of British jihadi bride Khadijah Dare, have been used in gory propaganda videos.

In the first case of its kind in 2016, a female Isis supporter saw her son taken into foster care after she returned to Britain.

Tareena Shakil, then 26, was jailed for six years for joining Isis and taking her toddler son with her.

What have other countries done with Isis fighters and families?

Like the UK, few countries are willing to take on the responsibility of bringing back citizens who left to join Isis. Most are worried that there will not be enough evidence to prosecute, and so they will be forced to release them despite the security risks.

So far, only the US and France have said they will bring back their citizens suspected of Isis membership, and prosecute them too.

The UK is calling on the US to prosecute the remaining members of the infamous “Beatles” execution squad, claiming there would not be enough evidence to prosecute them in the UK. That leaves open the possibility that El Shafee Elsheikh and Alexanda Kotey may be transferred to Guantanamo, or even face the death penalty.

What does the SDF want to happen with the British detainees?

The Syrian Democratic Forces, a Kurdish-led militia that has done the bulk of the fighting against Isis in Syria, has called repeatedly on the UK to take responsibility for its own citizens.

The group’s spokesperson, Mustafa Bali, has said that if Britain does not take them back then it may set up its own courts in the semi-autonomous region it now controls.