Video report by ITV News Security Editor Rohit Kachroo

Defence Secretary Sir Michael Fallon has told ITV News the UK faces a challenge to catch British IS fighters heading home from impending defeats in Syria and Iraq.

He said hunting down Britons who have been fighting for so-called Islamic State abroad is the "next stage of the campaign" against the shrinking but evolving terror group.

ITV News has learned at least one expat extremist had abandoned the battle as Kurdish fighters close in on victory over Isis in their former Syrian stronghold of Raqqa.

Correspondent John Ray, who is reporting from the frontline in Raqqa, was told the Brit had fled the city in the last few days and is thought to be heading home.

Questioned over the new threat by ITV News Security Editor Rohit Kachroo, Mr Fallon accepted catching returning extremists would become the next stage of the battle against Isis.

"Yes, we do need to track down those British citizens who have been fighting for (Isis)," he told ITV News.

"That's a criminal offence ... if we can establish evidence (of their fighting) then they can be charged and prosecuted if they return home."

Sir Michael Fallon said the UK would attempt to prosecute returning fighters. Credit: ITV News

Asked how Britain can keep track of the numbers returning, Sir Michael said security services would work with other countries while monitoring the UK's border controls.

"We make sure that they can't easily re-enter the UK but we're also working with neighbouring countries, Turkey for example."

He added: "Let me assure you if we can find them, we will detain them and where possible they will be charged."

Mohammed Emwazi, also known as Jihadi John, led a group of British IS executioners in Syria before being killed in 2015.

Sir Michael hailed the likely fall of Raqqa and Mosul in Iraq as physical progress in the wider fight against the ideology.

"It is important to destroy them physically to destroy the idea of a physical caliphate," he said.

"You can see how this poison can catch hold in Africa, in the Far East, in the Philippines, around the world."

Pressed by Kachroo, he said, "And here in the UK."

The recent deadly terror attacks in London and Manchester have come despite Isis shrinking abroad.

But Mr Fallon insisted: "We're winning this battle militarily. It's important to do both; to win the military campaign and then to go on and win the campaign against the extremists in our own country."