The US that the American military was 99 per cent certain that Mohammed Emwazi had been killed in a drone strike.
Downing Street and UK Ministry of Defence sources were marginally less certain in their response to the reports of his death than US sources, but added there was a "high degree of certainty that he has been killed".
Cameron said although it was not yet certain whether the strike had been successful, targeting Mohammed Emwazi was "the right thing to do".
If the raid killed the man seen in numerous ISIS beheading videos, "it will be a strike at the heart" of ISIS, Cameron said.
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He described Emwazi as an "ongoing and serious threat" to civilians around the world, particularly in Syria and the UK.
"He was IS'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 said the strike involved two MQ9 Reaper drones.
A formal statement from the Pentagon stopped short of asserting that Emwazi had definitely been killed, adding that it was assessing the operation.
Announcing details of the strike, Pentagon Press Secretary Peter Cook, said, "US forces conducted an airstrike in Raqqa, Syria, on November 12, 2015 targeting Mohamed Emwazi, also known as Jihadi John."
Emwazi participated in the videos showing the murders of US journalists Steven Sotloff and James Foley, US aid worker Abdul-Rahman Kassig, British aid workers David Haines and Alan Henning, Japanese journalist Kenji Goto, and a number of other hostages, the Pentagon said.