Fears Isis terrorists sneaking into Britain on small boats as terror watchdog issues alert

The independent reviewer of terrorism legislation Jonathan Hall added there was a possibility they could become involved in attack planning in the UK from abroad because of their knowledge of the country.

However, he admitted he did not know how plausible this risk was.

In 2024, the Office for National Statistics (ONS) reported 4,623 people from Syria had entered Britain via small boat, up from 2,268 in 2023.

The rapid collapse of the Kurdish-led Syrian Democratic Forces (SDF) in northeast Syria this week triggered uncertainty over the security of around a dozen prisons and detention camps.

In these camps, more than 10,000 Islamic State members and thousands more women and children with ties to the group are being held.

The SDF was backed by the US for years in the fight against Isis.

However, recent outbreaks of violence and chaos have led to jailbreaks at several detention camps, including the sprawling Al-Hol camp.

Asked about the risk of any Britons on the loose from the Al-Hol camp, Mr Hall told The Telegraph: "Could they return to the UK? In terms of the risk they present, one hopes that they have been identified and therefore are on a watchlist.

"Being on a watchlist is not a complete defence. They could come on a small boat or smuggle themselves in. That must be a risk.

"There is a question of the wider reaches of their influence. If they have experience of the UK, they could become involved in overseas attack planning based on their knowledge of the UK. I don’t know how plausible that is. If someone comes from Britain, they could say: 'Attack Britain. We know how best to do it.'"

Experts have insisted it is still very difficult for British-born devotees of Isis to re-enter the UK illegally.

They said anyone trying to come back to the UK faces the prospect of being detected by law enforcement agencies at any number of points along the route between Syria and western Europe.

All those who left Britain to join the caliphate are presumed to be on a watch list, which is likely to be circulated among partners on the continent.

This includes London-born Shamima Begum, who has such a high profile that it appears unlikely she could travel through Europe without being apprehended.

Anyone trying to come back to the UK would also struggle to obtain the large sums of money needed to pay smugglers for a place on a small boat.

Most British Isis members have had their UK citizenship revoked, meaning they can legally be refused entry.

While some of their children may still hold UK citizenship, those children would only be permitted to enter the UK without their parents.

Mr Hall warned the position of the children in the camps was "brutal", adding there was a risk they could be "hunted down and killed."

He continued: "There is always a danger of the children being radicalised and I am sure some of them may well have been.

"But the ethical point is very strong. They are entirely innocent. They were brought there by dreadful parents and it’s not their fault."

A Government spokesman said: "Our priority remains maintaining the safety and security of the UK. We will take action to prevent those who engage in activity that threatens our national security from entering the UK and robust security checks are in place.

"We remain committed to working with our partners in pursuit of the enduring defeat of Da’esh [Isis]."