In footage, which experts have said shows “child abuse of the most wicket sort”, the boy is seen dressed up in the black clothes generally worn by Isis fighters.

Given a large, apparently sharp-bladed hunting knife, the pre-school-aged child is urged on by a man off-screen, who shouts “takfir”, an Arabic term for a non-believer.

Syrian social media users condemned the video over the weekend, saying it represented a “childhood destroyed”.

The low-quality video was not thought to have been released directly by Isis’s media arm, and its provenance could not be confirmed. It was shared by an apparently Isis-supporting Twitter account however, with the title “The son of one of the Mujahideen Part 2”.

Julie Lenarz, a director at the foreign policy think-tank Human Security Centre, tweeted stills from the video and wrote: “Video emerges of Isis foreign fighter's toddler beheading a teddy bear. Child abuse of the most wicked sort.”

Isis has a long history of using children in its propaganda, and it claims to run “schools of jihad” for where boys under the age of ten are forced to watch images of beheadings and torture and use deadly weapons.

The UN has previously criticised Isis’s establishment of training camps “to recruit children into armed roles under the guise of education”.

It said the existence of such camps and the use of older children in military operations amounted to “a war crime”.

By Adam Withnall