Britain is to send Challenger 2 tanks to Ukraine to bolster the war effort against invading Russian forces.
Additional artillery support is also to be sent by the UK to assist the Kyiv government in the face of Kremlin aggression.
Sky News understands the UK will initially supply the Ukrainian military with around a dozen tanks – enough for a squadron.
The number is small but it is the first time a country has pledged western tanks to Ukraine.
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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.
The announcement comes after Sky News exclusively revealed the UK was considering supplying Ukraine with British tanks for the first time.
Ukrainian troops set to be trained in UK
UK government insiders said the decision had to be made now in order to ensure Ukrainian soldiers could be trained in how to use the advanced combat vehicles before the impending spring offensive.
Ministers hope the announcement will encourage other allies to follow suit.
Around 60 Ukrainian troops will likely be trained in the UK in how to operate the tanks, a source said.
The Ministry of Defence declined to comment and referred all questions on the announcement to Downing Street.
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PM's call with Zelenskyy
Following a call between Rishi Sunak and Ukrainian President Volodymyr Zelenskyy on Saturday, a Number 10 spokesperson said the two leaders had agreed on the "need to seize on this moment" after Ukrainian victories had "pushed Russian troops back".
The spokesperson added: "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.
"The prime minister stressed that he and the whole UK government would be working intensively with international partners to deliver rapidly the kind of support which will allow Ukraine to press their advantage, win this war and secure a lasting peace."
Mr Zelenskyy tweeted: "Always strong support of the UK is now impenetrable and ready for challenges. 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 UK only has 227 Challenger 2 tanks and many of them are not in a condition to be deployed.
Any tanks sent to Ukraine must be accompanied by ammunition, spare parts and the ability to be maintained.
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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 for Ukraine.
Germany also announced it would supply around 40 Marder armoured personnel carriers and France promised wheeled AMX-10 RC tank destroyers.
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.
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Last year, the UK sent 14 Challenger 2 tanks to Poland as part of a deal to help Ukraine by freeing up capacity for Warsaw to supply its Soviet-era T-72 tanks to Kyiv.
In service since 1994, the Challenger 2 tank weighs 62.5 tonnes and is armed with a 120mm rifled gun and a 7.62mm chain gun.