David Cameron is returning from his holiday in Cornwall after the murder of James Foley by a suspected British Islamist terrorist with the militant Isis organisation.
Downing Street announced the move after it emerged the murderer spoke with an English accent and amid renewed calls for parliament to be recalled to debate the escalating crisis.
Number 10 said: "If true, the brutal murder of James Foley is shocking and depraved.
"The prime minister is returning to Downing Street this morning. He will meet with the Foreign Secretary and senior officials from the Home Office, Foreign Office and the agencies to discuss the situation in Iraq and Syria and the threat posed by ISIL terrorists."
Foreign Secretary Phillip Hammond has echoed Cameron's warnings that Islamist militants posed a direct threat to the UK.
And the prime minister has previously expressed real concerns about UK citizens travelling to the region to join the fighters and then importing violence back into Britain.
So far he has resisted all calls to abandon his holiday or recall the Commons to debate the UK's growing involvement in Iraq but that pressure has now increased as a result of the latest atrocity.
The foreign secretary earlier said it appeared that "on the face of it" a Briton had carried out the execution of the freelance journalist.
And he repeated the warning about significant numbers of UK nationalists operating in Syria and Iraq.
"This may come as news to some people, but it certainly doesn't come as any news to us," said Hammond.
"We are absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with Isil and other extremist organisations.
"This is something we've been tracking and dealing with for many, many months. And I don't think this video changes anything. It just heightens awareness of the situation which is very grave and which we've been working on for many months."