The Defence Secretary stressed Britain’s priority is to ensure that militants trying to leave Iraq and Syria must not “pose a future threat to our national security”.
He threw his force behind international development minister Rory Stewart, who said at the weekend that the only way to deal with IS fugitives is to kill them “in almost every case”.
Sir Michael’s comments came as a report published yesterday suggested the UK has one of the largest numbers of returning IS [also known as Daesh] fighters.
The joint study by the Soufan Center and the US Global Strategy Network revealed about 425 jihadis with British links are heading home to the UK after being dispersed by Western-backed forces.
Sir Michael said: “I have made clear that those who have travelled to fight with Daesh in either Iraq or Syria will have been committing a criminal offence.
“Daesh is a proscribed organisation and we have to make sure that if they ever do return from Iraq and Syria, they do not pose a future threat to our national security.
“But they have made their choice, they have chosen to fight for an organisation that uses terror and the murder of civilians as a modus operandi.”
Mr Stewart had said at the weekend: “These people are a serious danger to us, and unfortunately the only way of dealing with them will be, in almost every case, to kill them.”
He added: “These are people who are executing people, who are torturing and murdering.”
Yesterday’s report on returning jihadis, written by former MI6 director of global counter terrorism Richard Barrett, said 850 British radicals are thought to have travelled to Syria.
About half will come back, it said.
This puts the UK behind only Turkey, Tunisia and Saudi Arabia in the league table of numbers of returning IS jihadis.
In total, about 40,000 fanatics travelled from their own countries to fight for the terror network in Syria and Iraq.
About 300 fighters are thought to have gone back to homes in Germany and 271 to France.
Earlier this month it was revealed that British jihadi Sally Jones – dubbed the “White Widow” – and her 12-year-old son Jojo were killed in a US Predator missile strike as they fled Raqqa.