Vladimir Putin should be put on trial and those who should committed war crimes in Ukraine "will be hunted down", Keir Starmer has said.
The Labour leader added the Western world needs to stand united in its condemnation of the invasion by Russia and the atrocities that have taken place.
During an interview on Good Morning Britain, Starmer said the Russian president should face a Nuremberg-style trial "as soon as possible" over the illegal conflict.
Presenters Richard Madeley and Kate Garraway quizzed Starmer about Putin's actions and the level of aggression used by his armed forces since the invasion in late February.
He replied: "He's a war criminal.
"This is Russian aggression. It's not only an unlawful invasion, for which Putin should stand trial - in my view.
"We should set up a tribunal, a Nuremberg-style tribunal so that he can stand trial for the invasion itself.
"On top of that, we have these awful war crimes being committed.
"It's very important, I know these things take time but every single person who's involved in that right now needs to know you will be hunted down for as long as you live and be brought to trial for what you have done.
"Let's all speak with one voice. Every single person involved in any way in those war crimes must be held to account and I know that's a common view, and that is a very good thing."
It comes as Boris Johnson issued a direct appeal to the Russian people to reject Putin's war in Ukraine, which he called a "stain" on their country's honour.
In a video message posted online, the Prime Minister urged Russians to download VPNs to enable them to circumvent the Kremlin's media controls and see for themselves the atrocities being committed in their name.
He said the "atrocities" committed by Russian forces - including the rape and massacre of innocent civilians - were so shocking that Putin had deliberately sought to hide the truth from his people.
"Your president knows that if you could see what was happening, you would not support his war," he said.
"He knows that these crimes betray the trust of every Russian mother who proudly waves goodbye to her son as he heads off to join the military.
"And he knows they are a stain on the honour of Russia itself. A stain that will only grow larger and more indelible every day this war continues."
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