Prime Minister David Cameron today described the apparent beheading of a US journalist by an Islamic State militant with a British accent as "shocking and depraved".
British police and Whitehall officials are trying to identify the man suspected of the killing.
Cameron said on Twitter: "If true, the murder of James Foley is shocking and depraved."
He cut short his holiday in Cornwall for meetings on Iraq and Syria with UK foreign secretary Philip Hammond, Downing Street said.
While Hammond acknowledged the militant spoke with an English accent, he said further analysis of the footage was needed.
Iraqi extremist group the Islamic State has posted a video showing the apparent execution of American journalist James Foley, and has threatened to kill a second reporter unless the US halts air strikes in Iraq.
Foley, 40, had been missing since he was seized in Syria in 2012.
Hammond said that the UK government was aware of "significant numbers" of British nationals involved with extremist organisations overseas.
He acknowledged the militant spoke with a British accent but said further analysis of the footage was needed.
"We're absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with Isil [now known as IS] and other extremist organisations," he said.
"This is something we have been tracking and dealing with for many many months and I don't think this video changes anything.
It just heightens awareness of a situation which is very grave and which we've been working on for many months," he added.
It is estimated that IS, which has seized large parts of northern Iraq and Syria, has up to 400 recruits from the UK in its ranks.
Some 69 people suspected of Syria-related jihadist activities have now been arrested in the UK.
