Mr Sunak planned to use the first session of the G20 summit on Tuesday to confront Russia’s Foreign Minister Sergey Lavrov, along with the UK's allies.
Russian President Vladimir Putin has decided to send Mr Lavrov in his place, as he continues to suffer military setbacks and unanimous condemnation from western powers.
“It’s telling that Putin is not there," Mr Sunak said on his flight to Indonesia.
“Russia is becoming a pariah state and he’s not there to take responsibility for what he’s doing.
“I’m going to take the opportunity to unequivocally condemn the Russian state and Putin for their abhorrent and illegal war.
Ukraine war latest - in pictures
“I will do that in the hall, I will do that if I see him elsewhere and that’s the right thing to do.”
The G20 leaders’ summit is the first since the Kremlin launched its full-scale invasion of Ukraine in February, and the first held in the shadow of a major European war instigated by one of its members.
The gathering looks likely to be dominated by the fallout from the conflict, which has caused and exacerbated global economic woes, driving up food and energy prices worldwide.
Ukrainian President Volodymyr Zelenskyy is set to address the event online.
Ukraine capital Kyiv hit by kamikaze drones - in pictures
Asked if he would encourage Kyiv to enter peace talks, Mr Sunak said that was a question for the Ukrainians.
“It’s a bit unfair to say to the Ukrainians: ‘look you should be negotiating’ when your country and your civilian infrastructure is being relentlessly bombed, as it is currently.
“So our job is to continue to help the Ukrainians defend themselves, and put themselves in the strongest possible position, at a time of their choosing, to bring a negotiated settlement.
“But nothing that Russia is doing at the moment would suggest that they are committed or interested in that approach right now.”