He told MPs “anyone who sends a Russian into battle to kill innocent Ukrainians” could be brought to court.

The prime minister added the UK is working with allies to set up a “particular international war crimes tribunal for those involved in war crimes in the Ukraine theatre”.

Speaking in the House of Commons, former United Nations commander Bob Stewart said he had given evidence for war crimes trials including where people were charged with genocide and crimes against humanity.

The Conservative MP for Beckenham said: “Could I ask (Mr Johnson) and the House to agree with me that any Russian who kills a Ukrainian must remember that one day they may well be brought to court for crimes against humanity or genocide?” Johnson replied: “Yes, and not just any Russian combatant but anyone who sends a Russian into battle to kill innocent Ukrainians.”