Boris Johnson has accused Russian President Vladimir Putin of committing war crimes in Ukraine.
At Prime Minister’s Questions, he said bombing innocent civilians “already fully qualifies as a war crime”.
He was responding to the Scottish National Party’s Ian Blackford, who called for Mr Putin to be prosecuted.
On Wednesday, International Criminal Court prosecutor Karim Khan said he was now investigating possible war crimes and crimes against humanity in Ukraine.
The investigation was launched after 38 nations grouped together to refer the situation to the prosecutor’s office, which said “collection of evidence” had started.
The UK government described the referral as the largest in the history of the court, which relies on co-operation with countries worldwide for support, particularly for making arrests.
Following the announcement, Mr Johnson said in a tweet: “We are crystal clear that Putin cannot commit these horrific acts with impunity.”
Ukraine’s president Volodymyr Zelensky has already accused Russia of war crimes after air strikes on the country’s second city, Kharkiv.
A week in to Russia’s invasion of Ukraine, attacks on key cities have intensified, with fighting raging in the north, east and south.
Russia attacks Ukraine: More coverage
On Tuesday, Mr Johnson described the tactics used by the Russian military under orders from Mr Putin as “barbaric and indiscriminate”.
“With every passing hour, the world is witnessing the horrors of Putin’s war in Ukraine,” said the SNP’s Westminster leader Mr Blackford. He called for Mr Putin to be prosecuted for the “full range” of war crime charges available.
Mr Johnson replied: “What we have seen already from Vladimir Putin’s regime in the use of the munitions that they have already been dropping on innocent civilians, in my view, already fully qualifies as a war crime.”
Justice Secretary Dominic Raab said the UK was “willing to provide the necessary technical assistance to support successful convictions” and stressed the importance of preserving “all evidence of war crimes”.
Asked about Mr Johnson’s remarks after PMQs, Downing Street said possible war crimes were occurring daily in Ukraine.
Mr Johnson’s official spokesman agreed the Russian attack on the Babyn Yar holocaust memorial in the Ukrainian capital Kyiv and the targeting of apartment blocks constituted war crimes.
The spokesman said: “Obviously, formally it will be for a criminal court to make that ruling but I think no one can be in any doubt that what we’re seeing daily, almost hourly now, are horrific acts that would certainly appear to be war crimes.”
Earlier, in a phone call, Mr Johnson and Mr Zelensky agreed sanctions must go further to exert maximum pressure on Russia.
The UK has partnered with Western allies to enact sweeping sanctions on Russia, including against Russian banks, businesses and individuals.
Meanwhile, the United Nations General Assembly has voted overwhelmingly to demand an immediate end to the invasion of Ukraine.
Just four countries – Belarus, North Korea, Eritrea and Syria – joined Russia in opposing a motion calling for the withdrawal of all occupying forces, while 35 nations abstained.
General Assembly resolutions are not legally binding but the move further isolates Russia diplomatically.
At the start of PMQs, MPs, many wearing the yellow and blue of the Ukrainian flag, gave a standing ovation in support of the Ukrainian ambassador, who was watching from the public gallery.
The applause, not usually permitted in Parliament, lasted for almost a minute.
Mr Johnson had opened the session by condemning Mr Putin’s “abhorrent assault on a sovereign nation”, adding the Russian leader had “gravely miscalculated” the Ukrainian people’s resolve to fight and the willingness of the “free world in standing up to his barbarism”.