Rishi Sunak has said Russia must "get out of Ukraine and end this barbaric war" as he confronted Vladimir Putin's officials at the G20 summit.
The prime minister made the comments in the first session on Tuesday as he criticised the absence of the Russian president at the talks on the Indonesian island of Bali.
"It is notable that Putin didn't feel able to join us here," he said. "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.
"The UK rejects this aggression. We will back Ukraine for as long as it takes."
Mr Putin chose not to attend the talks, with the Kremlin blaming scheduling conflicts, but observers say officials are seeking to shield him from condemnation by world leaders.
Mr Sunak also rebuked Russia by saying "countries should not invade their neighbours".
"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," he said.
"Surely these are things on which we can all agree."
Speaking via video link to the summit in Bali, Ukrainian president Volodymyr Zelenskyy, referred to it as "the G19" adhering to Kyiv's line that Russia should be excluded.
He called for the creation of a special tribunal to try Russian military and political figures for the crime of aggression against Ukraine, and the creation of an international mechanism to compensate Kyiv for wartime deaths and destruction.
"Everywhere, when we liberate our land, we see one thing - Russia leaves behind torture chambers and mass burials," he added.
"How many mass graves are there in the territory that still remains under the control of Russia?"
'Chorus of opposition to Putin'
Mr Sunak will use the summit to push the world's most powerful economies to do more to reduce their dependence on Russian exports, while supporting others to do the same.
He will also reiterate the UK's financial support for Ukraine, saying he committed £4.1bn in aid when he was chancellor.
This included £2.3bn in military aid, while Mr Sunak will promise to match this level of spending next year.
Before the meeting, he said: "Putin and his proxies will never have a legitimate seat at the table until they end their illegal war in Ukraine."
He continued: "At the G20, the Putin regime - which has stifled domestic dissent and fabricated a veneer of validity only through violence - will hear the chorus of global opposition to its actions."
Russia invaded Ukraine almost nine months ago, leaving the rest of the world struggling to deal with the fallout, which has included rising food and energy prices.
Many countries have stood firm, supporting Ukraine with weapons and aid, while imposing various sanctions on Russia.
Mr Putin sent Sergei Lavrov, the foreign minister, to the talks in his place.
He was taken to hospital on his arrival at the talks with a heart condition, the Indonesian authorities said - though the Russian foreign ministry dismissed the claims as "fake news".
Last week, the UK introduced legislation to stop countries using its maritime services to transport Russian oil unless it is purchased below a price cap - which Number 10 described as a "hugely influential measure, given the UK provides around 60% of global maritime insurance".