US forces have carried out a drone strike in Syria against the Islamic State group militant “Jihadi John”.
A US military source said there was a “high degree of certainty” he had been killed in Thursday’s attack near Raqqa.
PM David Cameron said although it was not yet certain whether the strike had been successful, targeting Mohammed Emwazi was “the right thing to do”.
The Kuwaiti-born British militant appeared in videos of the beheadings of Western hostages.
It is believed there was one other person in the vehicle targeted in the strike.
Speaking outside Downing Street, Mr Cameron said any successful attack would have been “a strike at the heart of Isil [Islamic State]”.
“It will demonstrate to those that would do Britain, our people and allies harm we have a long reach, we have unwavering determination and we never forget about our citizens,” he said.
He described Emwazi as an “ongoing and serious threat” to civilians around the world, particularly in Syria and the UK.
“He was Isil’s lead executioner and let us not forget he killed many, many Muslims too, and he was intent on murdering many more people.
“So this was an act of self-defence.”
The prime minister added that his thoughts were with the families of those “who were so brutally murdered”.
A senior US military source says the strike involved two MQ9 Reaper drones and took place at 20:50 GMT.
A formal statement from the Pentagon stopped short of asserting that Emwazi had definitely been killed, adding that it was assessing the operation.