David Cameron reiterated his condemnation of the "barbaric and brutal" killing of US journalist James Foley, calling it "an act of murder."
"Let's be clear about what this act is - it is an act of murder, and murder without any justification," the Prime Minister said.
"We have not identified the individual responsible on the video, but from what we've seen it looks increasingly likely that it is a British citizen - now this is deeply shocking," he added.
A British family have told ITV News of their devastation after discovering a close relative is fighting with the Islamic State in Iraq.
A French journalist who was held hostage with James Foley in Syria has told ITV News it was "devastating" to see his murder.