Rishi Sunak has urged world leaders to “double down” on support for Ukraine.
The Prime Minister said "the whole world must hold Russia to account" for war crimes against Ukraine and criticised the agreements of the "post-Cold War era" for failing Ukraine.
Calling for a strengthening of international law, he told the Munich security conference: "To win the peace, we also need to rebuild the international order on which our collective security depends.
"First, that means upholding international law. The whole world must hold Russia to account. We must see justice through the ICC for their sickening war crimes committed, whether in Bucha, Irpen, Mariupol or beyond, and Russia must also be held to account for the terrible destruction it has inflicted."
He added: "Second, the treaties and agreements of the post-Cold War era have failed Ukraine, so we need a new framework for its long-term security.
"From human rights to reckless nuclear threats from Georgia to Moldova, Russia has committed violation after violation against countries outside of the collective assurance of Nato."
The PM said a new Nato charter should be set up to help protect Ukraine from future Russian aggression.
"We must demonstrate that we'll remain by their side, willing and able to help them defend their country again and again,” he told delegates.
"Ukraine needs and deserves assurances of that support.
"So, ahead of the Nato summit in Vilnius, we will bring together our friends and allies to begin building those long-term assurances.
"Our aim should be to forge a new charter in Vilnius to help protect Ukraine from future Russian aggression."
Concluding his speech, he said the West has "no greater purpose" than to prove Ukrainian President Volodymyr Zelensky right in saying Ukraine is marching towards the most important victory of our lifetime.
The Prime Minister said: "What's at stake in this war is even greater than the security and sovereignty of one nation. It's about the security and sovereignty of every nation.
"Because Russia's invasion, its abhorrent war crimes and irresponsible nuclear rhetoric are symptomatic of a broader threat to everything we believe in.
"From the skies over North America to the suffering on the streets of Tehran, some would destabilise the order that has preserved peace and stability for 80 years.
"They must not prevail. And we need not be daunted.
"As President Zelensky said when he addressed the UK Parliament last week, we are marching towards the most important victory of our lifetime. It will be a victory over the very idea of war.
"We could have no greater purpose than to prove him right."
