Speaking on the plane to India, the Prime Minister criticised Russian president Vladimir Putin for refusing to attend the G20 summit in India – the second year in a row the Russian leader has missed the world leaders’ gathering.
Mr Putin has sent foreign minister Sergei Lavrov to New Delhi in his absence.
Mr Sunak said the Russian leader was steering clear of confrontation with the West following his invasion of Ukraine because he “doesn’t want to be held accountable”.
The British leader said he intends to highlight the impact that Moscow’s decision to collapse the Black Sea Grain Initiative is having on developing nations, which rely on cheap grain from Ukraine for food.
His pledge to use the gathering in New Delhi to continue to rally against the Kremlin’s attack on Kyiv comes despite host country India continuing to keep open ties with Mr Putin’s regime.
Mr Sunak, asked by reporters whether he had a message for Mr Lavrov at the G20, said: “When it comes to Lavrov, the message is, the fact that Putin is not there demonstrates his isolation in the global community.
“He wasn’t there last year, he is not there this year to answer for what he is doing.
“He doesn’t want to be held accountable.”
To mark Mr Sunak’s arrival in the Indian capital, Downing Street announced London will host a global food security summit in November.
No 10 said the food conference would be in response to Mr Putin in July opting not to renew the grain deal with Ukraine, which allowed cargo ships to transport grain out of the war-torn country’s Black Sea ports without fear of attack.
The UK Government has also announced British military and security services will monitor the Black Sea in a bid to deter Russia from striking cargo ships that are transporting grain from Ukraine to developing countries.
The Prime Minister said the loss of the grain pact was “causing an enormous amount of suffering to millions of people”.
He continued: “That initiative provided something like 30 million tonnes of food to over 45 countries who really needed it. And now it is not there.
“You’ve seen since he pulled out that dramatic increase in food prices.
“And in the last month alone, the Russians have destroyed more grain than would feed, I think, a million people for a year.
“Those are the consequences of what Russia is doing.
“They’ve also destroyed about a third of Ukraine’s civilian export capacity with their bombardment. That is wrong. It is not right.
“It is causing suffering in Ukraine, but it is also causing suffering to millions of people in very vulnerable countries around the world.
“And I want to do everything I can to hold them to account for that and to call out that behaviour.”
Chinese president Xi Jinping is another of the 20 leaders who chose not to fly out to India, with premier Li Qiang representing Beijing at the summit.
Mr Sunak would not commit to meeting with Mr Li while in New Delhi, saying only that he was “expecting to see a range of people”.
It comes after Foreign Secretary James Cleverly last month became the first UK foreign secretary for five years to visit China, in a trip that was criticised by hawkish Conservative MPs.
Mr Cleverly defended having a face-to-face meeting with vice president Han Zheng, who played a leading role in the anti-democracy crackdown in Hong Kong, saying regular meetings were important “to enhance understanding” and “to avoid misunderstanding”.