Britain was on Monday examining an Islamic State video showing a young boy in military fatigues and an older masked militant who both spoke with British accents.
The propaganda video, which could not be independently verified, also shows the killing of five men accused of spying for the West.
The masked man threatens Prime Minister David Cameron and vows that Islamic State will one day occupy Britain before shooting one of the alleged spies in the head.
After the killings of the five men, a young English-speaking boy, who is wearing a black bandanna and appears to be about four or five years old, is shown saying: "So go kill the kuffar right over there."
The United States said in November it had killed Mohammed Emwazi, a British citizen dubbed Jihadi John who became a symbol of Islamic State brutality after appearing in several hostage execution videos.
The masked militant shown in the new video was different from Emwazi but spoke in a clear English accent, waving a gun at the camera while criticizing Cameron.