Britain will send a squadron of 14 Challenger 2 tanks to Ukraine to help counter Russia's invasion "in the coming weeks", Downing Street says.
It marks the first time Western-made tanks have been pledged to Kyiv.
Around 30 AS90s, which are large, self-propelled guns, operated by five gunners, will also be provided.
It comes after Sky News exclusively revealed the UK was considering supplying Ukraine with British tanks.
The hope is that other allies will follow suit, most notably Germany, unlocking the Leopard 2 tank, which was manufactured in far greater numbers and is used by several other European countries.
Ukrainian troops to be trained in UK
The UK will begin training the Ukrainian Armed Forces to use the tanks and guns in the coming days, with more information set to be provided by Defence Secretary Ben Wallace on Monday.
Mr Wallace travels to Estonia and Germany this week to discuss NATO efforts to aid Ukraine, while James Cleverly, the foreign secretary, is heading to the US and Canada.
The provision of tanks and weapons is to help ensure "Ukraine wins this war", a Number 10 spokesperson said.
"The prime minister is clear that a long and static war only serves Russia's ends," they added.
"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."
PM's call with Zelenskyy
Following Saturday's call between Mr Sunak and Mr Zelenskyy, Ukraine's leader thanked the UK for its "impenetrable" support of his country's war effort.
"Always strong support of the UK is now impenetrable and ready for challenges," he tweeted.
"In a conversation with the prime minister, I thanked for the decisions that will not only strengthen us on the battlefield, but also send the right signal to other partners."
The Russian Embassy in London said the decision would drag out the confrontation, lead to more victims including civilians, and was evidence of "the increasingly obvious involvement of London in the conflict".
It added: "As for the Challenger 2 tanks, they are unlikely to help the Armed Forces of Ukraine turn the tide on the battlefield, but they will become a legitimate large target for the Russian artillery."
Earlier this week, Western officials warned Ukraine would not be able to take back significant territory from Russia without an increase in fighting power, including tanks and other heavy armour.
The US has promised 50 Bradley tank-killing armoured vehicles in its biggest military assistance package to date.
Many of the UK's 227 Challenger 2 tanks are not in a condition to be deployed, and the 14 being sent to Kyiv will be accompanied by ammunition, spare parts, and the ability to be maintained.
In service since 1994, it weighs 62.5 tonnes and is armed with a 120mm rifled gun and a 7.62mm chain gun.
Will anyone else send tanks?
Ukraine has long requested the mass-produced, German-made Leopard II tanks, used by several European allies, including Germany, Poland, Finland, the Netherlands, and Spain.
Warsaw and Helsinki have already signalled a willingness to supply their Leopard tanks to Kyiv, but this requires approval from Berlin because Germany holds the export licence.
Germany has announced it will supply around 40 Marder armoured personnel carriers, and France promised wheeled AMX-10 RC tank destroyers.