Rishi Sunak has demanded Vladimir Putin “get out of Ukraine and end this barbaric war”.
The PM was pictured glowering at Russian foreign minister Sergei Lavrov as they came face-to-face at the G20 summit in Indonesia.
In his address to world leaders, Mr Sunak rebuked Moscow by saying "countries should not invade their neighbours".
The gathering on the island of Bali is the first time a British PM has met a representative of the Kremlin since it started the war in Ukraine.
Putin has chosen not to attend the event and has sent his right hand man, Lavrov, instead.
Mr Sunak said: "It is notable that Putin didn't feel able to join us here.
"Maybe if he had, we could get on with sorting things out.
"Because the single biggest difference that anyone could make is for Russia to get out of Ukraine and end this barbaric war.
"That would be transformative for Ukraine, but for the world."
The PM added: "The UK rejects this aggression. We will back Ukraine for as long as it takes. And support peace on the basis of the UN Charter."
At the start of the two-day summit, Mr Sunak acknowledged it was taking place amid "challenging circumstances".
"The context for this G20 is stark," the PM told the plenary session. "Russia's illegal invasion of Ukraine has profound implications for us all, because it has undermined the fundamental principles of sovereignty and territorial integrity.
"We all depend on these principles. They are the foundations of the international order. They must be upheld.
"It is very simple - countries should not invade their neighbours, they should not attack civilian infrastructure and civilian populations and they should not threaten nuclear escalation.
"Surely these are things on which we can all agree."
Mr Sunak argued that global economic challenges have been made "much, much worse" by Russia's invasion of Ukraine, Rishi Sunak told fellow world leaders at the G20 summit.
"The weaponisation of energy and food is totally unacceptable,” he went on.
"Two thirds of Ukraine's grain goes to developing countries, yet Russia has destroyed grain stores and blocked shipments.
"It is harming the most vulnerable people around the world. And this has nothing to do with sanctions.”
The PM called on Moscow to allow grain shipments out of Ukraine to continue as it threatens to stop them from Saturday.
Criticising Putin for shunning the Bali summit, Mr Sunak added: "One man has the power to change all of this."
The G20 is made up of 19 nations and the EU. The member countries are Argentina, Australia, Brazil, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey, the UK and the US.
Mr Sunak is expected to hold one-on-one talks with several of the leaders including Mr Biden, who he will meet for the first time.
