Prime Minister David Cameron was on Wednesday to chair an emergency meeting following the execution of a second US journalist by Islamist fighters in Iraq and the threat that a British hostage will be next.
In a video showing the severed head of 31-year-old Steven Sotloff, a masked militant warned a British man, widely identified as David Cawthorne Haines, would be killed in response to US air strikes against militants in northern Iraq.
Speaking ahead of the meeting of Britain's emergency response committee, Cameron condemned Sotloff's apparent beheading as "a despicable and barbaric murder".
The video posted by the Islamic State came just two weeks after its killing of US reporter James Foley.
"As I have said consistently over the last few weeks, ISIL terrorists speak for no religion. They threaten Syrians, Iraqis, Americans and British people alike and make no distinction between Muslims, Christians or any other faith," Cameron said.
Defence Secretary Michael Fallon and former foreign secretary William Hague were seen arriving at Downing Street for the 8:15am (0715 GMT) meeting, which will address the broad threat posed by the extremist group and the government's possible response.
Britain has so far not joined in US air strikes against the Islamist fighters, but has helped armed Kurds fighting in northern Iraq and has dropped aid to people surrounded by IS fighters on Mount Sinjar and in the town of Amerli.
Cameron on Monday announced that Britain will make it easier for police to seize passports from would-be jihadist fighters, increase air travel checks and tighten controls on the movement of suspected radicals.
The prime minister was addressing the House of Commons after Britain on Friday raised its terrorism threat level to "severe" because of concern about possible plots by radical Islamists returning from Iraq and Syria with battlefield experience.