Facing Russian Foreign Minister Sergei Lavrov in the plenary hall, the Prime Minister called on Moscow to “get out of Ukraine and end this barbaric war” as he blamed the conflict for worsening global economic challenges.
He criticised Russian President Vladimir Putin for shunning the meeting, saying: “Maybe if he had, we could get on with sorting things out”.
The two-day gathering of leaders of the world’s major economies was opened by Indonesian President Joko Widodo, who said the world would struggle to move forward “if the war does not end”.
The annual summit comes as G20 nations are deeply divided over Russia’s actions in Ukraine, which have pushed up food and energy prices around the world.
In his remarks, Mr Sunak said the the context for this G20 “is stark”. According to a Downing Street transcript of his speech to the closed session, the Prime Minister said: “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.”
He said the economic issues “we should be focusing on today are made much, much worse” by Moscow’s actions.
“The weaponisation of energy and food is totally unacceptable,” he said, adding that Russia is “harming the most vulnerable people around the world” by destroying grain stores and blocking shipments.
Mr Sunak urged fellow leaders to support the renewal of a deal allowing grain exports from Ukraine’s Black Sea ports to ease the global food crisis.
He continued: “One man has the power to change all of this.
“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 Prime Minister said he “rejects this aggression” as he vowed to “back Ukraine for as long as it takes.”
He praised Volodymyr Zelensky’s “incredible fortitude” after the Ukrainian President addressed the Bali summit via videolink at the invitation of the Indonesian hosts.
Mr Sunak is holding a series of bilateral meetings on the margins of the event, which began with drinks and nibbles with his Canadian counterpart Justin Trudeau soon after the Prime Minister’s arrival on the tropical island on Monday.
More controversially, he is on Tuesday set to meet with Saudi Arabia’s Crown Prince Mohammed bin Salman, who is widely believed to be implicated in the 2018 murder of Saudi journalist Jamal Khashoggi.
He also has one-on-one talks lined up with US President Joe Biden, Indonesia’s Mr Widodo, India’s Prime Minister Narendra Modi, Japan’s premier Fumio Kishida, and new Australian Prime Minister Anthony Albanese on Wednesday.
Mr Sunak will return to the UK on Thursday in time for his budget, in which Jeremy Hunt could impose up to £60 billion in tax rises and spending cuts.