British Prime Minister Rishi Sunak on Saturday pledged to provide 14 Challenger 2 tanks to Ukraine, making it the first Western country to supply the heavy tanks Kyiv has been calling for.
The pledge saw a swift reaction from Russia which warned it would only "intensify" the conflict.
"Bringing tanks to the conflict zone, far from drawing the hostilities to a close, will only serve to intensify combat operations, generating more casualties, including among the civilian population", the Russian embassy in the UK said.
Sunak said the tanks were a sign of the UK's "ambition to intensify our support to Ukraine", according to a readout of a phone call with Ukrainian President Volodymyr Zelensky.
Ukraine's European allies have sent Kyiv more than 300 modernised Soviet tanks since Russia invaded in February 2022.
But they have so far held off on dispatching the Western-made heavy tanks that Ukraine has repeatedly requested to push forward against Russian invaders.
Zelensky thanked the UK on Twitter for making decisions that "will not only strengthen us on the battlefield, but also send the right signal to other partners".
Ukraine's forces have taken heavy losses in the battles of Soledar and Bakhmut in recent months and have called on the country's allies to give it more support.
Russia said on Friday its forces had wrested control of the war-scarred town of Soledar in east Ukraine, its first claim of victory in months of battlefield setbacks, although Ukraine said fierce fighting was still under way.
"To win this war, we need more military equipment, heavy equipment," Andriy Yermak, head of Ukraine's presidential office, said.
Foreign Minister Dmytro Kuleba also said that he had spoken with US Secretary of State Antony Blinken and "emphasised the need" for Ukraine to receive Western-type tanks.
The UK will also send around 30 AS90s, large, self-propelled guns, operated by five gunners.
Fears of escalation
The tanks will arrive in Ukraine in coming weeks, a second statement from Sunak's Downing Street office said later Saturday, adding that the UK will also begin training the Ukrainian Armed Forces to use the tanks and guns in coming days.
"As the people of Ukraine approach their second year living under relentless Russian bombardment, the prime minister is dedicated to ensuring Ukraine wins this war," a Downing Street spokesman said.
"The prime minister is clear that a long and static war only serves Russia's ends. That's why he and his ministers will be speaking to our allies across the world in the days and weeks ahead to ramp up pressure on Putin and secure a better future for Ukraine."
The issue of heavy tanks has long been a key one for Kyiv and many experts see providing Ukraine with modern tanks as a vital building block in its ability to win against Russia.
Germany has been especially hesitant of supplying heavy tanks.
It has delivered powerful mobile artillery and air defences but remains fearful of an escalation with Moscow if its tanks face off directly against their Russian opposite numbers.
Sunak was committed to working with partners to ensure Ukraine was "in the strongest possible place to enter future peace negotiations from a position of military, economic and diplomatic strength and secure a strong and lasting peace", the statement added.
Britain's announcement comes ahead of next week's meeting of the Ukraine Defence Contact Group, which coordinates arms supplies to Kyiv, at Ramstein Air Base in Germany.