Rishi Sunak has confirmed that Ukraine will shortly receive the first British Challenger 2 tanks, piling pressure on Germany to follow suit.
The move makes the UK the first Western power to supply Ukraine with main battle tanks.
A squadron of 14 Challenger 2s will be sent in the coming weeks and the UK will begin training the Ukrainian army to use them this week, Downing Street said.
Mr Sunak said that the modern tanks were needed to "seize the moment" and to "push back Russian troops" in a phone call with Volodymyr Zelensky, the Ukrainian president, on Saturday.
"The Prime Minister outlined the UK's ambition to intensify our support to Ukraine, including through the provision of Challenger 2 tanks and additional artillery systems," a Downing Street spokesman said.
Around 30 AS90s - large, self-propelled guns operated by five gunners - are also being sent. Further details are set to be outlined in the House of Commons on Monday.
UK defence and security officials believe there is a window of opportunity with Russia on the backfoot due to resupply issues and plummeting morale.
The tanks and weapons are "the start of a gear change in the UK’s support", according to Downing Street.
Mr Zelensky thanked Britain for its "strong support" and said that other countries should follow Mr Sunak’s lead.
Poland and Finland have both pledged to send German-made Leopard II tanks to Ukraine, but they need permission from Olaf Scholz, the German Chancellor, before they can complete the transfers.
Mr Scholz has dithered on greenlighting the move because he does not want Germany to get dragged deeper into the war, but has faced significant criticism for dragging his feet.
Modern Western tanks are expected to be a game-changer for Ukraine’s armed forces.
Around 300 modernised Soviet-era T-72 tanks have already been handed over to Ukraine from its neighbours, such as the Czech Republic and Poland, but Western tanks had been considered a step too far and likely to escalate the conflict.
Russia’s foreign ministry downplayed the impact of the British Challenger II tanks.
"As we’ve said previously, weapons supplies are legitimate targets for Russian strikes," said Maria Zakharova, the Russian foreign ministry's spokeswoman.