Cameron confirmed Haines' death in a statement late yesterday after the British Foreign Office had said earlier that it was "working urgently to verify the video." Haines is the third Westerner beheaded in recent weeks by the Islamic State group, which has seized vast swaths of territory in Syria and Iraq.
"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil," Cameron said, adding that "my heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal."
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," Cameron said. Cameron returned to his residence at 10 Downing Street shortly after midnight and is expected to chair a meeting of the government's emergency response committee early today.
The video emerged a day after Haines' family issued a public plea late Friday urging his captors to contact them. The Foreign Office said it was offering Haines' family "every support possible. They ask to be left alone at this time."
Islamic State militants recently beheaded two American journalists, James Foley and Steven Sotloff, and posted the videos online after the US began launching airstrikes and humanitarian missions in August to aid waning Iraqi and Kurdish security forces in northern Iraq.
The Islamic State group has also put out videos showing the beheading of Kurdish and Lebanese soldiers and the mass shooting of dozens of captured Syrian soldiers. At the end of the video showing the beheading of Sotloff, the Islamic State group threatened to kill Haines next and briefly showed him on camera.