The Prime Minister will deploy his ministers on an international diplomatic blitz to galvanise support for Ukraine after agreeing to provide British tanks to Kyiv.
Rishi Sunak signed off on sending British Army Challenger 2 main battle tanks to the frontline in eastern Europe during a phone call with Ukrainian President Volodymyr Zelensky on Saturday.
Downing Street has since confirmed that 14 tanks will be handed over as part of the agreement and around 30 AS90s – large, self-propelled guns operated by five gunners – are expected to follow.
Mr Sunak has also asked the Foreign Secretary and Defence Secretary to meet with allies across the globe in a bid to encourage them to follow suit in providing Ukraine with support in the battle against Russia.
No 10 said the Challenger 2 tanks pledge represented a "gear change in the UK's support" for Kyiv.
The training in Britain of Ukrainian armed forces on how to use the tanks and AS90 guns will start in the coming days.
Mr Zelensky, writing on Twitter, said the move would "not only strengthen us on the battlefield but also send the right signal to other partners".
Officials said the decision to embark on a global diplomatic flurry was taken after the Prime Minister "analysed the military picture".
He is said to have concluded that Ukraine would need early assistance if it was to push Russia back following President Vladimir Putin's stalled and protracted invasion.
It comes amid reports that both Kyiv and Moscow are gearing up for a spring offensive when the weather is warmer.
Downing Street said Mr Sunak recognises that a "long and static war only serves Russia's ends".
A No 10 spokeswoman said: "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.
"Alongside his closest military advisors, he has analysed the military picture, looked at the strategic impact of the UK's support and identified a window where he thinks the UK and its allies can have maximum impact.
"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."
According to No 10 officials, UK defence and security experts believe a window of opportunity has opened up, with Russia on the backfoot due to resupply issues and plummeting troop morale.
That analysis encouraged the Prime Minister to use the coming days to try and persuade allies to deploy their planned support for this year "as soon as possible" in order to have "maximum impact", the officials said.
Mr Sunak will send Foreign Secretary James Cleverly to the US and Canada next week to work together on "galvanising further international action" in support of Kyiv.
Defence Secretary Ben Wallace, after setting out the UK's latest military support to Mr Zelensky's forces in the Commons on Monday, is set to meet Nato allies in Estonia and Germany.
The British diplomatic efforts come as the Ukraine-Russia conflict approaches its first anniversary.
Responding to Mr Sunak's decision to send tanks to the battlefield, the Russian embassy in the UK said it would "generate more casualties, including among the civilian population".
The comments come despite the targeting of civilian infrastructure by the Kremlin's military during the conflict and allegations of human rights abuses in Russian-captured areas.
The embassy said the Challenger 2 tanks would "become legitimate large-scale targets for the Russian forces".