Only one in 10 jihadis returning from Syria to the UK have been prosecuted, it can be revealed.

Amid calls for the government to repatriate Shamima Begum for criminal investigation rather than remove her British citizenship, police have warned it is “no easy task” to evidence a suspect’s activities abroad.

Ben Wallace, the security minister, said around 40 people “have been successfully prosecuted so far – either because of direct action they have carried out in Syria or, subsequent to coming back, linked to that foreign fighting”.

But more than 400 people “of national security concern” are believed to have returned from conflicts in Syria and Iraq.

The Home Office would not provide a breakdown of what offences the returnees were charged with or what groups they fought for, but the total includes at least one man convicted of trying to join Isis.

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Mr Wallace announced the figures during a parliamentary debate over expanded terror laws, which the government hopes will make prosecuting returning jihadis easier.

Of more than 900 people who travelled to Syria and Iraq from the UK, he said at least 20 per cent are believed to have been killed.

The Independent understands Ms Begum and her baby are among around 20 British women and children being held in Syrian camps, along with six suspected fighters.

Speaking to journalists on Tuesday, Metropolitan Police commissioner Cressida Dick admitted that security services do not know how many of Isis’s surviving British recruits intend to return.

Ms Dick said evidence of a criminal or terrorist offence was needed to prosecute, adding: “The very fact of going is not an offence.

“Some people returned from that area in the early days who had almost certainly done nothing other than humanitarian aid work. We talked to them and assessed their risk ... Many people have come back and just gone on with peaceful lives.”

Speaking in parliament on Wednesday, home secretary Sajid Javid said all Isis fighters who re-entered the UK had been investigated and “the majority have been assessed to pose no or a low security risk”.

None of the four fatal terror attacks launched in London and Manchester in 2017 were carried out by returnees from Iraq and Syria, although Manchester bomber Salman Abedi had fought in Libya.

Shamima Begum's child could retain British citizenship, admits Sajid Javid

And the Parsons Green bomber had been forced into an Isis training camp as a child, before coming to the UK as a refugee from Iraq.

Assistant commissioner Neil Basu, the head of UK counterterror policing, said security services had previously seen foreign fighters as the biggest risk, but “the threat was already here”.

But he said security services had been planning for returning jihadis and are “using a wide range of measures and powers available to us to mitigate this threat”.

“Anyone who returns from Syria or other conflict zones, having gone in support of any proscribed terrorist group – whether that’s fighting for or against Isis – or for any other illegal purposes, can expect to be investigated by the police,” he added.

“This is to determine if individuals have committed any terrorist or other criminal offences, regardless of their motivation, and to ensure that they do not pose a danger to the public or the UK’s national security.”

Mr Basu previously told The Independent he was concerned that “people will either get back without our knowledge, because that’s entirely possible, or that we will not have a case we can prosecute”.

“Proving what someone has been doing in a theatre of war is no easy task,” he added. “We’re very lucky, we’ve only seen a trickle of people [returning from Syria]. Most of these people don’t want to come back here.”

Raffaello Pantucci, the director of international security studies at Rusi, said that British jihadis involved in previous conflicts like Afghanistan and Bosnia had moved to other foreign battlefields.

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“Some do head back home but not all do – we’re probably going to see odd cases turning up over the next decade or so,” he told The Independent.

“A lot of the returnees went and came back from Syria very quickly, and some were possibly disillusioned ... Those that were dedicated enough to stay have the potential to be more dangerous.

“The danger is not only that they might come back and do something, but that they can build future networks that might produce other groups and more potential terrorists.”

Convicted terrorists can be put through the Home Office’s Desistance and Disengagement Programme in an effort to deradicalise them, but Mr Pantucci said its success was “very difficult” to gauge.

While some returnees are still being monitored by police and intelligence services, others are not under active surveillance or restrictions.

The most stringent measures – Terrorism Prevention and Investigation Measures (Tpims) – are only in force for eight people according to the most recent government statistics.

The Home Office spent almost £5m on 23 of the orders, figures obtained by The Times revealed, including on accommodation, poling and surveillance.

A spokesperson for the Crown Prosecution Service said: “When there is enough evidence, we won’t hesitate to prosecute cases such as those involving alleged Isis fighters returning from Syria or other countries.