Buildings at Zaporizhzhia nuclear plant, the largest in Europe, were damaged after it was hit by shelling in a move branded “reckless” and condemned around the world.
The US Embassy in Ukraine said attacking a nuclear plant was a war crime.
Ukraine-Russia: ‘My mother is teaching English to children in her Kyiv bomb shel…
Justice secretary Dominic Raab called it “an affront to the world at large”, while Linda Thomas-Greenfield, the US ambassador to the United Nations, told an emergency meeting: “By the grace of God, the world narrowly averted a nuclear catastrophe last [Thursday] night.”
Elsewhere, images laid bare the scale of the destruction caused by Russian shelling in the northern Ukrainian city of Chernihiv.
Residential areas in several cities have come under bombardment.
Mr Raab said the Zaporizhzhia offensive meant more than ever the international community needed to “come down hard on Putin”.
He told Times Radio: “It is clearly reckless, irresponsible and not only the fact they were shooting, bombarding that particular site, but when the Ukrainian emergency authorities were trying to put out the fire, the shelling continued. It must stop.
“We support the Ukrainians in dealing with the security situation there, but also I think come down hard on Vladimir Putin.”
Mr Raab, who is also Deputy Prime Minister, said the Russian president was “at risk” of ending up in prison for war crimes.
He said: “I think it is a very real risk that I must now contemplate.
“Beyond his personal situation, every commander operating in Ukraine, or indeed Moscow, when they are faced with illegal orders, whether it is to target civilians or otherwise, attack illegal or unlawful sites, they now know the ICC [International Criminal Court] is investigating and the chief prosecutor Karim Khan is, I believe, traveling to Ukraine.
“They must now know that they face the very real risk of ending up in the dock of a court and, ultimately, in a prison if they follow through on those illegal, unlawful orders.”
Mr Raab warned Mr Putin could “resort to ever more barbaric measures as he gets frustrated”.
Defense Secretary Ben Wallace said there was a risk of a “new Cold War” and that “a new Iron Curtain will descend upon Europe”.
He said Mr Putin was “playing with fire” by attacking the Zaporizhzhia nuclear site.
Officials said the site was now likely to be under Russian control, but with Ukrainian staff still on the ground to maintain safety.
Speaking at a press conference in Copenhagen, Mr Wallace said the move was “incredibly dangerous”.
He said: “It’s not just dangerous for Ukraine and Russia, it’s dangerous for Europe, and it is playing with fire that really is beyond anything that has to do with logic or necessity.”
The attack was being discussed on Friday at an emergency meeting of the United Nations Security Council, which Mr Johnson had called for in the early hours of the morning following a conversation with Ukrainian president Volodymyr Zelensky.
A Downing Street spokeswoman said the two leaders had agreed a ceasefire was “crucial”.
She said: “The Prime Minister said the reckless actions of President Putin could now directly threaten the safety of all of Europe. He said the UK would do everything it could to ensure the situation did not deteriorate further.”
The UK’s permanent representative to the UN said Russia “must keep fighting away from and protect the safety and security of nuclear sites”.
Dame Barbara Woodward said: “This is the first time that a state has attacked a fueled and functioning nuclear power plant.
“International law special protection for nuclear facilities and it is difficult to see how Russia’s required actions were compatible with its commitments under Article 56 of the additional protocol of the (Geneva) Conventions. It must not happen again.”
Elsewhere, Commander Richard Smith, head of the Metropolitan Police’s Counter Terrorism Command, which includes the War Crimes Team, announced it was now gathering evidence.
He said: “Following the news that the ICC had opened an investigation into war crimes in Ukraine, our War Crimes Team is now seeking to gather any evidence that might be present here in the UK of such crimes in Ukraine.
“This could be in the form of direct messages, images or videos that friends or relatives here in the UK have been sent by those in Ukraine.
“Or it could be somebody who was previously in Ukraine and who may have witnessed or even been a victim of a war crime and has since traveled to the UK.”
According to the International Atomic Energy Agency (IAEA), the UN’s nuclear energy watchdog, the fire at Zaporizhzhia did not cause a change in the plant’s radiation level and nor had the shelling or fire caused any damage to “essential” equipment.
Rafael Mariano Grossi, director-general of the IAEA, said he has volunteered to travel to Chernobyl – the site of a nuclear disaster in the 1980s and now under Russian control since the invasion started – to reach a “framework” to uphold nuclear safety.
Western officials say Ukraine continues to hold the majority of the country’s main cities, but there has been a high level of air and artillery attacks, including those hitting civilian sites.
They believe the resistance of the Ukrainians was having a psychological impact on Russian troops, and the lack of progress made by the Kremlin’s forces had forced senior commanders into the front of the battle, resulting in some being killed.
On Friday, home secretary Priti Patel met with families, women, and children on the Polish border in Medyka as she confirmed the immediate and extended family members of British nationals and people settled in the UK could stay in the country for three years.
It came as the BBC announced it was “temporarily suspending” the work of all its news journalists and support staff in Russia after authorities passed legislation cracking down on foreign outlets.
Director-General Tim Davie said the new law appeared to “criminalize the process of independent journalism” in the country.
Former prime minister Gordon Brown said the prospect of Mr Putin ending up in the dock for the crime of aggression against Ukraine was “a realistic option”.
The former Labor leader called on countries to support the creation of a special court to punish the Russian leader.
Meanwhile, the Scottish Government has announced it will not take part in any fisheries negotiations with the Russian Federation.
It also confirmed ventilators, bandages and syringe pumps are being sent to Ukraine as part of the second shipment of medical supplies and equipment donated by NHS Scotland.
Charities thanked people for donating to the Disasters Emergency Committee appeal for Ukraine after it raised more than £6 million in Scotland in under 24 hours.