Security sources have told Sky News that more than four hundred British jihadists who travelled to the Middle East to fight for terror organisations like Islamic State and the al-Nusra Front have returned home, and most have evaded the authorities.

Former Scotland Yard Specialist Firearms Officer and author Tony Long told the broadcaster these fighters could pose serious problems for the British authorities.

“These are combat-hardened soldiers,” he said. “They might not be trained in the way that NATO might train their soldiers but they’ve seen more close quarter conflict and more urban fighting than probably most members of the British Armed Forces and you have to respect that.

“Of course they’re bringing that knowledge back with them to the UK and it’s very, very difficult because of the legal restrictions that are put on the security services and the police to actually monitor all of these people.”

The true number of fighters in Britain could be far higher than the estimate broadcast by Sky News, however.

The Home Office told the Daily Express that around 375 jihadists had made their way back to Britain as long ago as December 2015; given the significant reverses inflicted on terror groups in the Middle East by the Russian-backed Syrian government and the Western-backed Iraqi government since then, it seems unlikely the increase in returnees would be so small.

Speaking to The Telegraph in 2016, former head of the National Counter Terrorism Security Office Chris Phillips said: “I don’t believe the UK knows how many people have left for Syria or indeed come back.

“There are many ways of getting back into the UK avoiding checks, including bus routes and ferry crossings. What we have to avoid is a false sense of security just because we have a stretch of water between us and Europe.”

Henry Bolton OBE, former head of the Borders Unit at the Organisation for Security and Co-operation in Europe (OSCE), told Breitbart London that Britain’s maritime borders were particularly at risk, as “successive governments have dismantled the layered mosaic of border security” which used to protect the country’s coast.