The UK's security and intelligence agencies are examining a video by so-called Islamic State, to identify a man and young boy with British accents.
The video purportedly shows the killing of five men who IS says were spying for the UK.
The authorities will also be carrying out voice-print analysis to match what is heard against individuals known to have travelled to Iraq and Syria.
The man threatens attacks in the UK, during the 10-minute film.
He also says the footage, which has not been independently verified, is a message for Prime Minister David Cameron.
After the apparent killings the young boy, who seems to be aged about six or seven years old and is wearing military-style clothing, is seen pointing into the distance and talking about killing "unbelievers".
'Break borders'In the video, the masked man who has a British accent and is holding a gun, mocks Mr Cameron for daring to "challenge the might" of the extremist group.
He goes on to say: "We will continue to wage jihad, break borders and one day invade your land where we will rule by the Sharia."
The five men, wearing jumpsuits and kneeling in a desert location, then appear to be shot in the back of the head, after making what is claimed to be their confessions.
One of the men says he had been asked to provide information about the location of IS militants, including two Britons, apparently to help target them with air strikes.
Some of the five say they are from Raqqa in Syria while another says he is from Benghazi, Libya, but none say they are from the UK.
A UK Foreign Office spokesman said: "We are aware of the video and are examining its content."
'Jihadi John'IS, also referred to as Daesh, has previously released propaganda videos showing killings, including footage showing the apparent beheading of two US journalists, James Foley and Steven Sotloff, and two British aid workers David Haines and Alan Henning.
Mohammed Emwazi, the Briton who became known as Jihadi John, appeared in the videos. He was killed in a drone strike in Syria in November.
BBC security correspondent Frank Gardner says it would "probably be a fairly short-lived attempt... to take on the mantle of Mohammed Emwazi".
He says the UK government is unlikely to say who they think he is or confirm whether the men killed in the footage were working for Britain.
"IS is absolutely paranoid about [informants]," our correspondent says.
"They are looking over their shoulders, wondering where the next drone strike is coming from and they are wondering who in their midst is informing and reporting on them."
AnalysisBy Gordon Corera, BBC security correspondent
It is less than two months since British IS propagandist Mohammed Emwazi, the man known as Jihadi John, was killed by a drone strike in Syria.
This latest video by IS seems to show another man trying to take up his mantle.
The identity of the masked man is so far unknown but authorities will be trying to identify him and confirm if he is British.
Emwazi was killed after intelligence - most likely from informers - located him in a vehicle in Raqqa, Syria.
The fact that this latest video shows the killing of alleged spies indicates the extent to which IS is trying to track down those who might be providing information.
Children also regularly feature in IS propaganda videos and the brief appearance of a child - apparently with a British accent - at the end of the video will also most likely be of interest to the authorities.
Abu al-Furat, a member of a Raqqa-based group which opposes IS, told BBC Radio 4's Today programme they doubted the killed men were spying for Britain.
"Daesh [IS] has been harmed very much after the death of some British citizens who are fighting with Daesh," he said.
"And in this video, the executor is trying to copy John [Emwazi]. They want to say that if John died, there are others that can do this job."
Margaret Gilmore, senior associate fellow at security think tank the Royal United Services Institute, told BBC Radio 5 live the video had "an air of desperation".
"They are trying to detract from recent setbacks," she said, highlighting recent losses in Iraq.
'IS brand'But Shiraz Maher, senior research fellow at the International Centre for the Study of Radicalisation at Kings College London, told Today the group remained "very powerful".
Jonathan Russell, head of policy at the counter-extremist think tank, the Quilliam Foundation, said it was "shocking" to hear British voices and to see a child in the video, but that both were being used "to reinforce the IS brand".
At least 700 people from the UK have travelled to support or fight for jihadist organisations in Syria and Iraq, British police say. About half have since returned to Britain.
The IS group, notorious for its brutality, seized large swathes of territory in Syria and Iraq in 2014, when it formally declared the establishment of a "caliphate" - a state governed in accordance with Islamic law, or Sharia, by God's deputy on Earth, or caliph.