The video features a masked militant speaking in what appears to be a British accent, threatening the U.K. and deriding Prime Minister David Cameron before five captives are shot in the head.
Cameron dismissed the video as "desperate stuff" from an "appalling" terror group that is "losing ground."
"The fact that we are a successful, tolerant, democratic, multi-faith, multi-ethnic nation — they hate that," he said. "But I know that Britain will never be cowed by this sort of terrorism ... they will be defeated."
Both his office and Britain's Foreign Office said they were examining the contents of the video.
They declined further comment but its similarities to "Jihadi John" propaganda videos — execution-style murders, taunts against the West — were clear.
U.S. officials said in November they were "reasonably certain" that an airstrike had killed Mohammed Emwazi, who was dubbed "Jihadi John" by hostages who nicknamed him and three British colleagues "The Beatles."
Emwazi was a Kuwait-born British citizen in his 20s. He appeared masked and wielding a knife in a series of ISIS propaganda videos featuring the beheadings of Western hostages, including Americans Steven Sotloff and James Foley. He was also known for being especially rough with hostages, sometimes beating and torturing them.
The new ISIS video — released by the militant group's official media office in Raqqa and circulated online Sunday — differs in that the captives are shot, not beheaded.
However, the militant who speaks to the camera sounds British and hurls similar jibes: He calls Cameron an "imbecile," critiques the Western campaign against ISIS and warns one day Britain will be defeated by the militants.
The captives — described as foreign spies — are dressed in orange garb reminiscent of uniforms worn by detainees at Guantanamo Bay that are similar to those in previous ISIS propaganda videos. They issue what appear to be forced "confessions" and later are shown kneeling in front of masked ISIS fighters before being shot.
Local British media seized on the similarities between the previous ISIS killer and latest propaganda video and reported that intelligence agencies were chasing leads on who the new masked murderer might be.
"Spies shot by new Jihadi John," read a headline in the Sun newspaper, while the Daily Telegraph echoed with "Hunt for the new Jihadi John."