Rishi Sunak said Vladimir Putin had shown “utter contempt” for the international rules-based order with his brutal war on Ukraine.
Speaking at the G20 summit in Bali, the British Prime Minister hailed the spirit of co-operation between leaders in response to challenges caused by Russia’s invasion of its neighbour.
He skirted around questions about Tuesday night’s missile strike on Poland, which killed two people, saying “it is important to establish the facts”.
Ukraine’s President Volodymyr Zelenskyy blamed Moscow for the blast in the Nato member state but did not provide any evidence.
US President Joe Biden said it was “unlikely” that the offending missile was launched by Russian forces.
Russia's Defence Ministry on Wednesday said the explosion was caused by a Ukrainian air defence missile and that Russian strikes in Ukraine had been no closer than 35km from the Polish border. The rocket struck a grain plant only 6km from Poland's border with Ukraine.
Mr Sunak had strong words for Mr Putin, saying there is no corner of the world unaffected by his “cruel and unrelenting” war.
“The persistent threat to our security and global asphyxiation has been driven by the actions of the one man unwilling to be at this summit – Vladimir Putin,” he said.
“There isn’t a single person in the world who has not felt the impact of Putin’s war.”
Mr Sunak said the UK and its allies were working to determine the cause of the blast in Poland.
“What we agreed this morning is that it is important to establish the facts and this is exactly what is happening as we speak,” he said. "We will get to the bottom of what happened.
“That was happening at a time when the G20 was gathered trying to find resolution to some of the world’s challenges. I think it shows utter contempt for the international rules-based system.”
As the world waits for answers on who launched the missile, Mr Sunak said: “We should all be clear: none of this would be happening if it weren’t for Russia’s invasion of Ukraine.
“This is the cruel and unrelenting reality of Putin’s war.”
While world leaders were working together to address international challenges, he said, Mr Putin was launching “indiscriminate attacks on civilians in Ukraine”.
After the fatal blast in Poland, Mr Sunak said allies had held emergency talks “to underscore our solidarity with Ukraine and Poland”.
The Prime Minister said he had offered Polish President Andrzej Duda his “wholehearted support” and assurance that the UK “stands steadfastly behind him and his people at this worrying time”.
As long as the war drags on, he said “it poses a threat to our security and that of our allies” and it “will continue to devastate the global economy”.
The Russian Defence Ministry denied accusations its army was behind the attack.
In a statement released via the RIA news agency, the ministry said photos of the wreckage were “unequivocally identified by Russian defence industry specialists as elements of an anti-aircraft guided missile of the S-300 air defence system of the Ukrainian Air Force”.
Earlier, the Polish President said his country lacked concrete evidence to determine who fired the weapon that struck the grain site.
A Nato source said Mr Biden had informed G7 and Nato partners that the blast had been caused by a Ukrainian air defence missile.
In a tweet, Mr Biden said he had spoken to Mr Duda "to express my deep condolences for the loss of life in eastern Poland and offer our full support for Poland's investigation of the explosion".
"We will remain in close touch to determine appropriate next steps as it proceeds," he said.
Explosion near Polish-Ukrainian border - in pictures
