Prime Minister David Cameron is to chair a meeting of the government's emergency response committee on Sunday to discuss an Islamic State (IS) video showing the beheading of British aid worker David Haines.
IS militants fighting in Iraq and Syria released the video on Saturday. Reuters could not immediately verify the footage, but the images were consistent with those of the filmed executions of two American journalists, James Foley and Steven Sotloff, in the past month.
Cameron, who returned to London ahead of schedule on Saturday night, had called the murder an act of pure evil and vowed to bring the aid worker's killers to justice.
"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil. My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal," he said in a statement.
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
Haines's purported executioner appears to be the same man who featured in videos with Foley and Sotloff. The man, nicknamed "Jihadi John" by Western media, seems to have a British accent.
Security services in Britain have been trying to identify him using voice recognition technology. At the end of the same video, another hostage is shown and threatened.
Cameron has faced calls from some of his Conservative party lawmakers to authorize British air strikes against IS and has said he is ruling nothing out apart from putting combat troops on the ground.
Instead, Britain has confined itself to delivering humanitarian aid, carrying out surveillance, arming Kurdish forces who are fighting IS militants and promising training in Iraq.
On military action, London supports U.S. air strikes while keeping its own options open.
Mike Haines, brother of the slain aid worker, paid tribute to his sibling on Sunday, saying David had decided humanitarian work was the field he wanted to work in, but he had been murdered in cold blood.
"He was and is loved by all his family and will be missed terribly," Mike Haines said in a statement released through the British Foreign Office. He said his brother, a Scot born in 1970, left behind two daughters from two marriages.
Alex Salmond, Scotland's First Minister, said Haines's murder was an "unspeakable act of barbarism."
"It should be remembered that Mr Haines was in the region as an aid worker helping local people. His murder will be totally condemned by all people with any sense of humanity," said Salmond.
