US forces have carried out a drone strike in Syria against the Islamic State militant known as “Jihadi John”.
A US military source said there was a “high degree of certainty” he had been killed in Thursday’s attack in Raqqa.
Mohammed Emwazi, a Kuwaiti-born British militant, appeared in videos of the beheadings of Western hostages.
UK PM David Cameron said although it was not yet certain whether the strike had been successful, targeting Emwazi was “the right thing to do”.
It is believed there was one other person in the vehicle targeted in the strike.
Speaking outside Downing Street, Mr Cameron said the UK had been working with the United States “literally around the clock” to track Emwazi down.
He said any successful attack would have been “a strike at the heart” of Islamic State (IS), and would demonstrate to those who would harm Britain that “we have a long reach, we have unwavering determination and we never forget about our citizens”.
He described Emwazi as an “ongoing and serious threat” to civilians around the world, particularly in Syria and the UK.
Emwazi appeared in beheading videos of victims including British aid worker David Haines and taxi driver Alan Henning.
Mr Haines’s daughter Bethany said she felt an “instant sense of relief” upon hearing reports of Emwazi’s possible death, “knowing he wouldn’t appear in anymore horrific videos”
“He was only a pawn in Isis’s stupid game, but knowing it’s over, that he’s finally dead, still hasn’t sunk in,” she said in a statement.
“As much as I wanted him dead I also wanted answers as to why he did it, why my dad, how did it make a difference?”
The Henning family were informed on Thursday night by a government contact that the strike had taken place, the BBC understands.
Stuart Henning, Mr Henning’s nephew, wrote on Twitter: “Mixed feelings today wanted the coward behind the mask to suffer the way Alan and his friends did but also glad it’s been destroyed.”
Labour leader Jeremy Corbyn said it appeared Emwazi had been “held to account for his callous and brutal crimes”.
“However, it would have been far better for us all if he had been held to account in a court of law,” he said.
On Twitter, a group called Raqqa is Being Slaughtered Silently posted a map claiming to show the location of the drone strike.
It added: “The good thing is #JihadiJohn got killed near Clock tower where #ISIS Execute people.”
A senior US military source said 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.
by Frank Gardner, BBC security correspondent
As the militant who sadistically murdered Western aid workers and journalists on camera, Mohammed Emwazi became a top target for US and British intelligence agencies, even though he is thought to have played no military role within Islamic State.
After his identity was revealed in February, Emwazi largely stayed out of sight, taking particular care not to leave a digital trail to his whereabouts.
But GCHQ, the UK government’s communications headquarters, has expended enormous efforts to intercept and decipher any encrypted messages that might reveal his location or those of his associates.
Emwazi is believed to have travelled to Syria in 2013 and later joined IS militants.
He first appeared in a video in August last year, when footage was posted online showing the murder of US journalist James Foley.
He later appeared in videos of the beheadings of US journalist Steven Sotloff,Mr Haines, Mr Henning, as well as American aid worker Abdul-Rahman Kassig,also known as Peter, and Japanese journalist Kenji Goto.