Rishi Sunak: Ukraine's long-term security must be ensured now
Mr Sunak told the Munich Security Conference that allies must give the country "advanced, Nato-standard capabilities".
He said now was the time to "double down" on military support.
Throughout the conference, Ukraine's allies have reiterated the case for defending the country.
The three-day gathering to discuss global security, taking place in Germany ahead of the first anniversary of Russia's invasion, provided a key test of Western support for Kyiv as both sides in the war prepare for spring offensives.
Ukraine's allies tried to demonstrate their resolve and tried 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.
Most of those who attended the conference - from heads of state and ministers to diplomats and spies - were from Europe or the US, including US Vice-President Kamala Harris and nearly 30 European heads of government. No Russian officials were invited.
Mr Sunak had said he wanted to "make sure other countries follow our lead" in providing battle tanks, and training soldiers and aviators on Nato-standard aircrafts.
In his speech in Germany, he said: "Ukraine needs more artillery, armoured vehicles and air defences, so now is the time to double down on our military support.
"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 said allies "must demonstrate that we'll remain by their side, willing and able to help them defend their country again and again".
He went on to say that, as well as having a military strategy "to gain a decisive advantage on the battlefield", allies also needed "to rebuild the international order on which our collective security depends".
Mr Sunak said international law needed to be upheld in order to hold Russia to account. He also called for "a new framework" for Ukraine's long-term security, and said the international community's response had not been strong enough against Russia's aggression.
Before his speech, Mr Sunak met German Chancellor Olaf Scholz and both agreed on the need to sustain "the record level of international support for Ukraine", a Downing Street spokeswoman said.
The unspoken question in Munich was what will happen if the participants meet this time next year and the war is still going on.
Of particular concern was 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.
The UK is to start training Ukrainian forces to fly Nato-standard jets and Mr Sunak has said "nothing is off the table".
But Defence Secretary Ben Wallace has said there will be no immediate transfer of UK fighter jets to Ukraine.
He told the BBC it could take months to train pilots and the UK was instead focused on using alternative provision of air cover to Ukraine.
Some Nato member countries are also worried that giving jets to Ukraine would be viewed as escalating the war, risking direct confrontation between the Western military alliance and Russia.
Since Russia invaded on 24 February last year, the UK has spent £2.3bn on military assistance, making the country the second biggest donor behind the US. The government has said it plans to match this spending again this year.
Military equipment provided by the UK so far includes tanks, air defence systems and artillery.
However, Kyiv has become increasingly frustrated by the time Western weapons have taken to arrive. Deliveries of battle tanks - promised last month by countries including Germany, the US and the UK - are still thought to be weeks away from arriving on the battlefield.
During Mr Sunak's meeting with Ms Von der Leyen, the pair also had what Downing Street described as a "positive discussion" about fixing issues with the Northern Ireland Protocol.
They agreed there had been "very good progress to find solutions" but that "intensive work in the coming days is still needed" to get a deal on post-Brexit trading arrangements over the line, according to No 10.
It comes after sources suggested that a deal could be reached between the UK and the European Union as early as next week, after more than a year of negotiations.