ITV News Senior International Correspondent John Irvine reports on the latest developments after a missile killed two people in Poland
The prime minister, who was speaking at a press conference, said he spoke with Poland President Andrzej Duda on Wednesday to offer his "wholehearted support".
On Tuesday, Russia fired 90 missiles and initiated more than 10 attack drone strikes in Ukraine, leading to some accusations that the weapon which landed in Poland was fired from Russia.
However, Poland has said the missile does not appear to be an intentional attack by Russia.
Prime Minister Rishi Sunak told reporters the latest round of missile strikes in Ukraine were part of the 'cruel and unrelenting reality of Putin's war'
The NATO ally instead said it was likely that the projectile came from air defences in neighbouring Ukraine, as it sought to defend itself from Russia's aerial bombardment.
"Ukraine’s defence was launching their missiles in various directions and it is highly probable that one of these missiles unfortunately fell on Polish territory," said President Duda.
"There is nothing, absolutely nothing, to suggest that it was an intentional attack on Poland."
NATO itself said it echoed the Polish analysis, with its Secretary-General, Jens Stoltenberg, saying: "We have no indication that this was the result of a deliberate attack."
Nonetheless, Mr Stoltenberg and others laid overall, but not specific, blame on Russian President Vladimir Putin’s war.
"This is not Ukraine’s fault. Russia bears ultimate responsibility," Mr Stoltenberg added.
Ukrainian President Volodymyr Zelenskyy has denied his country was involved in the strike and has demanded access to the investigation.
Speaking on Good Morning Britain, ITV News Global Security Editor Rohit Kachroo discusses the potential consequences of a reported Russian missile strike that killed two people inside Poland
Elsewhere, Foreign Secretary James Cleverly said the UK will provide any support needed in the effort to investigate the missile strike.
He told the House of Commons that Poland "will lead the investigation to establish exactly what has happened and the UK stands ready to provide any practical or technical assistance".
Before Poland and NATO announced their findings, US President Joe Biden had said it was "unlikely" the missile came from Russia.
Meanwhile, UK Defence Secretary Ben Wallace told ITV News that NATO would "progress once we know what's happened".
UK Defence Secretary Ben Wallace told ITV News the government had reinforced its 'unquestionable' solidarity with Poland
Shortly after the attack was first reported, world leaders at the Bali G20 summit held an emergency meeting on Tuesday.
Alongside Mr Sunak and Mr Biden, French President Emmanuel Macron, German Chancellor Olaf Scholz, Japan’s premier Fumio Kishida and Canada’s Justin Trudeau all attended the morning roundtable.
Each of them pledged to offer their "full support" for Poland’s investigation into the incident.
Mr Sunak described the latest attacks as "the cruel and unrelenting reality of Putin's war", insisting that "as long as it goes on it poses a threat to our security and that of our allies".
He added: "As last night's events laid bare, the challenges we face are utterly political.
"The persistent threat to our security and global economic asphyxiation has been driven by the actions of the one man unwilling to be at this summit, Vladimir Putin. There is not a single person in the world who hasn't felt the impact of Putin's war."
'As last night's events laid bare, the challenges we face are utterly political'
The prime minister also said Chancellor Jeremy Hunt would build on the "international foundations" agreed at the G20 when he makes the Autumn Statement on Thursday.
"By promoting free markets, forging strong international relationships and prioritising our stability and security we will build a global platform for the United Kingdom to thrive," he said.
Following the emergency meeting at the G20 summit, Mr Sunak then held his first in-person talks with the US president, in which he hailed the US as the UK’s "closest ally".
The PM told Mr Biden: "I think the partnership between our countries is unique and enduring.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know
"The United States is our closest ally, biggest trading partner, closest security partner.
"And I’m glad to see that partnership in action doing good whether it’s standing up to - and I agree with your words - barbaric activity by Russia in Ukraine, but also tackling climate change, stabilising the global economy."
Meanwhile, Mr Sunak and his Canadian counterpart Justin Trudeau spoke to Ukrainian President Zelenskyy from the G20 in Indonesia.
"The prime minister and Prime Minister Trudeau expressed their condolences for the terrible loss of life as a result of Russian attacks on Ukraine overnight," a Downing Street spokesman said.