Prime Minister Rishi Sunak is to urge world leaders to "double down" on military support for Ukraine.
In a speech at the Munich Security Conference, Mr Sunak will say allies must give the country "advanced, Nato-standard capabilities".
But he will also go further and call for Nato to offer long-term security assurances against Russian aggression.
The underlying theme of the conference is an attempt by allies to remake the case for defending Ukraine.
The three-day gathering to discuss global security, taking place in Germany a few days ahead of the first anniversary of Russia's invasion, will provide a key test of Western support for Kyiv as both sides in the war prepare for spring offensives.
Ukraine's allies are trying to demonstrate their resolve, trying to convince the Russian government that they will not give up or give in, even if the cost in "blood and treasure" increases in coming months as both sides launch fresh military campaigns.
Most of those attending the conference - from heads of state and ministers to diplomats and spies - are from Europe or the US, including US Vice-President Kamala Harris and nearly 30 European heads of government. No Russian officials have been invited.
The challenges of giving fighter jets to UkraineWhat weapons are being supplied to Ukraine?
Ahead of the conference, the UK's prime minister said the summit was "an opportunity for me to talk to our partners and allies around the world about our efforts to increase our support to Ukraine so they can defeat Russian aggression".
Speaking from Downing Street, Mr Sunak said he wanted "to make sure other countries follow our lead" in providing battle tanks, and training soldiers and aviators on Nato-standard aircraft.
In his speech in Germany, he is expected to say: "When Putin started this war, he gambled that our resolve would falter. Even now he is betting we will lose our nerve.
"But we proved him wrong then, and we will prove him wrong now."
Calling for a new Nato charter to provide assurances of long-term support, Mr Sunak will also say allies "must demonstrate that we'll remain by their side, willing and able to help them defend their country again and again".
The unspoken question in Munich is what will happen if the participants meet this time next year and the war is still going on.
Of particular concern is whether the political and economic costs of the war could prove too much to bear, as the Russian leadership assumes, or the western alliance will stand firm behind Ukraine.
The uncertainty around these issues is another reason why allies want to step up support now, to ensure Ukraine can see off any Russian offensive and launch a counter-attack on its own. President Volodymyr Zelensky is not the only one urging speed.
Last week, the Ukrainian leader visited the UK, as well as Paris and Brussels, where he appealed for European leaders to supply his country with modern fighter jets.