Rishi Sunak has told Russia "to get out of Ukraine and end this barbaric war".
Rishi Sunak has issued a strong rebuke to Russia at the G20 summit
The UK Prime Minister confronted Vladimir Putin's officials at the opening session of the G20 summit in Bali and made the hard-hitting comments about the ongoing conflict.
Putin has not travelled to the Indonesian island for the event and is being represented by the Kremlin's foreign minister Sergey Lavrov.
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.
"The UK rejects the aggression. We will back Ukraine for as long as it takes."
Sunak, who took over as Prime Minister last month after the short-lived Downing Street tenure of Liz Truss, delivered another stinging rebuke to Russia by stating that "countries should not invade their neighbours".
He said: "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."
Sunak had vowed that Putin's regime would hear the worldwide condemnation of their actions in Ukraine at the summit.
Prior to 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.
"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."