Henry Dare, also known as Sunday, said that he recognised the child from the film as Isa, one of the sons of his daughter Khadijah, who has also been used as a figurehead by Isis.

Isa has been previously used for propaganda purposes when a posting on social media account linked to his mother showed him holding an AK-47 rifle.

Speaking to Channel 4 News from his home in south-east London, Dare said: “I can’t disown him. He’s my grandson. I know him very well.” There is no independent verification of his claim.

The child appeared at the end of the 11-minute video that surfaced on Sunday. He was wearing military fatigues and warned in English: “We are going to go kill the kafir [non-believers] over there.”

The video apparently showed the murder of five men and was fronted by a masked man with a British sounding accent, saying that he had a “message for Cameron” and threatened attacks in the UK.

Dare accused Isis of “just using a small boy”. He added: “He doesn’t know anything. He’s a small boy. They are just using him as a shield.”

When asked whether he had spoken to his grandson on the phone, Dare said: “Well, he doesn’t like it over there,” referring to where he is believed to be, in an Isis-held area of Syria.

Khadijah Dare, who grew up in Lewisham to Nigerian Christian parents, converted to Islam as a teenager before leaving for Syria. In 2014, she posted a photograph on her personal Twitter account of her then four-year-old son Isa, meaning Jesus in Arabic, smiling with an AK-47 rifle.

Videos in which she talks about her new life in Syria, practises firing a Kalashnikov or appears with her young son and jihadi husband, have been in the frontline of the propaganda war.

 Either her account, or those linked to her, are known for posting pro-Isis messages on various social media sites encouraging other young women to make the journey to the war zone and she is one of the first known western women to have travelled to Syria.

He said he had recently spoken to his daughter, who was christened Grace before she converted to Islam, “weeks ago, when she called me”. “I keep on ignoring her calls because she has brought shame to our family and to herself,” he said.

Dare comes from a devout Christian household. In 1987, her family moved to Britain from Nigeria. Her mother, Victoria Dare, said in an interview with the BBC last year that her daughter had been someone who had previously been zealous in practising her Christian faith. “She loves church. She was the one who was dragging me: ‘Mum I found a good church again.’”

At college in London, she studied media studies, film studies, psychology and sociology, and enjoyed watching football on television. But her life turned, her parents say, when her behaviour began to change – the then teenager one day announced she had converted to Islam and changed her name.

Just before Grace turned 18, she came home and made an announcement, according to her mother. “She just said: ‘I’m now a Muslim,” her mother recalled and said she reacted by saying: “Muslim? What for? What happened?”

When she got to Syria, she met and married Abu Bakr, a fighter from Sweden, in a wedding believed to have been arranged by his mother.

Broadcast footage has shown her and her new militant husband playing with her son in Isis-controlled territory by an inflatable paddling pool. They compare their firearms, with her husband saying: “My Kalash is better than yours”. Khadijah replies: “No, it’s not.” She was expecting a child with her new husband at the time.

She speaks in her London accent about her marriage. “Alhamdulillah, [praise to God] I couldn’t find anyone in the UK who was willing to sacrifice their life in this world for the life in the hereafter. I prayed and Allah ruled that I came here to marry Abu Bakr,” she says to the camera, standing next to her new husband who is holding her young child. He is believed to have been later killed in fighting.

Lewisham council said: “We are unable to confirm the identity of the boy in the video. The matter is clearly one on which the police are leading. The council is liaising with the police and we are deeply concerned about any suggestion of a link between these abhorrent acts and our community.”

The Metropolitan police said in August that more than 30 children from the UK had been made the subject of family court orders over fears they might be radicalised . At that time, judges had considered cases involving 12 families.

Assistant commissioner Mark Rowley, the country’s most senior counter-terrorism officer, saidsome children were “almost babes in arms”, with ages ranging from two or three up to 16 or 17.