Boris Johnson condemned Vladimir Putin’s “cruelty and barbarism” after a missile strike on a shopping centre in Ukraine left scores feared dead.
The Prime Minister said the attack, on the day Ukraine’s Volodymyr Zelensky addressed the G7 summit, would strengthen the resolve of allies to resist Mr Putin.
Mr Zelensky, who had urged G7 leaders to supply missile defence systems, described the toll of the attack on the site in Kremenchuk as “unimaginable”.
Mr Johnson said: “This appalling attack has shown once again the depths of cruelty and barbarism to which the Russian leader will sink.
“Once again our thoughts are with the families of innocent victims in Ukraine.
“Putin must realise that his behaviour will do nothing but strengthen the resolve of the Ukraine and every other G7 country to stand by the Ukraine for as long as it takes.”
The massacre at the mall followed days of Russian missile strikes on Ukrainian cities, including the capital Kyiv, as Mr Zelensky prepared to address the G7 remotely.
Mr Johnson is said to have told fellow leaders that it was “stupid of Putin” to attack Kyiv “when all of us are in the same place because it is only going to make us feel more resolute and united”.
Earlier, the Prime Minister said the “price of freedom is worth paying” and the UK must be prepared to support Ukraine’s fight against Russia for as long as it takes despite the cost.
The conflict in Ukraine has added to the rising cost of living by exacerbating turbulence in international energy prices and causing food shortages due to supplies of grain being prevented from leaving the country’s ports by Russia’s Black Sea fleet.
But speaking at the G7 summit in Germany, Mr Johnson said those pressures will start to ease and the long-term economic impact of defending the rules-based system of international conduct will be beneficial to the global economy.
If Mr Putin is not resisted, it could give the green light to countries such as China to pursue their own goals of territorial expansion, he suggested.
The UK has so far contributed around £1.5 billion of economic and humanitarian support to Ukraine plus some £1.3 billion of military assistance.
The Prime Minister told the BBC at the summit in the Bavarian Alps: “I think that the economic impacts on the UK will start to abate, we’ll find ways around things and some of the cost pressures will start to come down.
“But just in terms of staying the course, imagine if you didn’t.
“Imagine if we allowed Putin to get away with the violent acquisition of huge chunks of another country, a sovereign, independent territory, the lessons for that would be absolutely chilling in all of the countries of the former Soviet Union, you can see what’s happening in the Baltic countries already.
“But the read across would also be felt in east Asia, as well.
“So, in terms of the economic effects of that, that would mean long-term instability, it would mean anxiety across the world.”
Comparing the situation to the defeat of Nazi Germany, Mr Johnson declined to put a limit on UK support.
“The point I would make to people is, I think that sometimes the price of freedom is worth paying.
“And just remember, it took the democracies, in the middle of the last century, a long time to recognise that they had to resist tyranny and aggression.
“It took them a long time, it was very expensive.
“But what it bought in the end, with the defeat of the dictators, particularly of Nazi Germany, it bought decades and decades of stability, a world order that relied on a rules-based international system.
“And that is worth protecting, that is worth defending, that delivers long-term prosperity.”
Mr Johnson has been struck by the unity on show at the G7 amid concerns that a protracted conflict could lead to “fatigue” among leaders and populations.
There had been concerns that France’s Emmanuel Macron – who has repeatedly held talks with Mr Putin and warned that any peace deal must not leave Russia humiliated – had been wavering in support for a lengthy war.
But Downing Street insisted there is no dispute between the UK and France over the issue, with the two leaders on the same page over Ukraine and their friendly relationship characterised as “Le Bromance” by No 10 aides.
A new Anglo-French summit is planned to build on the relationship.
Mr Zelensky is understood to have told G7 leaders not to let the conflict in his country “drag on over winter” – a season where the frozen ground could give Russian armour an advantage.
He told the leaders: “If Ukraine wins, you all win.”
And in a sign that he is not willing to back down and accept a peace deal that gives up swathes of Ukraine to Russia, he said: “We will only negotiate from a position of strength.”
In a joint statement, the G7 said: “We will continue to provide financial, humanitarian, military and diplomatic support and stand with Ukraine for as long as it takes.
“As we do so, we commit to demonstrate global responsibility and solidarity through working to address the international impacts of Russia’s aggression, especially on the most vulnerable.”