David Cameron has described the suspected killing of Jihadi John as an "act of self defence" and "the right thing to do" as he thanked the United States for carrying out the strike on the Isis executioner.
He said that if the targeted drone strike was successful, his death will be "a strike at the heart of Isil".
However he added that it was not yet known whether Mohammed Emwazi is dead and warned that until his death is confirmed, he still posed a threat to innocent civilians around the world.
The Isis executioner, who Mr Cameron branded a "barbaric murderer", has been hunted by British and American intelligence services ever since videos emerged of him beheading hostages in Isis-controlled terrirtory.
Mr Cameron said the thoughts of the country were with the families of those who had been "brutally murdered" by Mr Emwazi, including the British aid workers David Haines and Alan Henning.
Delivering a statement outside Number 10 Downing Street, Mr Cameron said: "If this strike was successful, and we still await confirmation of that, it will be a strike at the heart of Isil," Mr Cameron said outside Downing Street.
"And it will demonstrate to those who would do Britain, our people and our allies harm: we have a long reach, we have unwavering determination and we never forget about our citizens."
Here is Mr Cameron's statement in full:
Good morning. Last night, the United States carried out an air strike in Raqqa, Syria, targeting Mohammed Emwazi – the ISIL executioner known as Jihadi John.
We cannot yet be certain if the strike was successful.
But let me be clear. I have always said that we would do whatever was necessary, whatever it took, to track down Emwazi and stop him taking the lives of others.
We have been working, with the United States, literally around the clock to track him down.
This was a combined effort. And the contribution of both our countries was essential.
Emwazi is a barbaric murderer. He was shown in those sickening videos of the beheadings of British aid workers.
He posed an ongoing and serious threat to innocent civilians not only in Syria, but around the world, and in the United Kingdom too.
He was ISIL’s lead executioner, and let us never forget that he killed many, many, Muslims too.
And he was intent on murdering many more people.
So this was an act of self-defence. It was the right thing to do.
Today I want to thank the United States: the United Kingdom has no better friend or ally.
And I want to pay tribute to all those professionals in our own security and intelligence agencies and Armed Forces for the extraordinary work they do on behalf of our country.
On this, as so often, they've been working hand in glove with their American colleagues.
We are proud of them.
If this strike was successful, and we still await confirmation of that, it will be a strike at the heart of ISIL.
And it will demonstrate to those who would do Britain, our people and our allies harm: we have a long reach, we have unwavering determination and we never forget about our citizens.
The threat ISIL pose continues.
Britain and her allies will not rest until we have defeated this evil terrorist death cult, and the poisonous ideology on which it feeds.
Today though, my thoughts, and the thoughts of our country, are with the families of those who were so brutally murdered.
Japanese citizens Kenji Goto and Haruna Yukawa, American journalists Steven Sotloff and James Foley and aid worker Abdul-Rahman Kassig.
And of course our own citizens. Aid workers David Haines and Alan Henning.
Nothing will bring back David and Alan.
Their courage and selflessness stand in stark contrast to the empty callousness of their murderers.
Their families and their friends should be proud of them, as we are.
They were the best of British and they will be remembered long after the murderers of ISIL are forgotten.
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