Boris Johnson has signalled Russian President Vladimir Putin could face war crimes charges over the invasion of Ukraine.
The Prime Minister told MPs "anyone who sends a Russian into battle to kill innocent Ukrainians" could be brought to court.
He also told MPs 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?"
"Yes, and not just any Russian combatant but anyone who sends a Russian into battle to kill innocent Ukrainians," Mr Johnson replied.
Watch: Tactical map - How Russia's invasion has played out. Thumbnail image: Angela Bragato/Alamy Stock Photo.
Labour MP Chris Bryant also claimed Mr Putin "must be brought to a court of law and end his days in prison".
"The Prime Minister is absolutely right, we equivocated shamelessly after Crimea," Mr Bryant said.
"We were spineless. We must not be spineless now because what will inevitably happen is either the Baltic states, or one of the members of NATO, or perhaps Sweden or Finland will feel the wrath of Putin next and that will mean British action," he said.
"He must be brought to a court of law and end his days in prison, mustn't he?"
Mr Johnson noted that "one of the most fascinating things about what Putin is doing is how close an analogy there is between his actions and those of Slobodan Milosevic".
"Exactly the same nonsense being peddled about the mystical union between Kosovo and Belgrade, between Kyiv and Moscow, exactly the same aggression, and remember that Slobodan Milosevic died on trial," he said.