Rishi Sunak has vowed that Ukraine "will never be alone" as the UK announced £2.5bn of military aid to Ukraine over the coming year.
It is the UK's largest annual commitment since Russia's full invasion of Ukraine in February 2022.
The PM made the announcement as he met President Volodymyr Zelensky in Kyiv and signed the new agreement.
Mr Sunak said that the support was vital because if Vladimir Putin "wins in Ukraine, he will not stop there".
Officials said the package will provide Ukraine with long-range missiles, air defence and artillery shells. Some £200m will be spent on drones.
They said the military package - for the next financial year beginning in April - would result in the largest delivery of drones, most of which will be UK-made, to Ukraine by any country.
Significantly, the prime minister has decided not to make a financial commitment lasting several years.
Some ministers and senior military figures had argued privately this would send a stronger signal to Moscow of Britain's long-term support.
Instead, Mr Sunak has chosen to spend £200m more than the last two years, when the UK's annual military commitment to Ukraine was worth £2.3bn.
Downing Street said the package of support would form the first step in what it called "an unshakeable hundred-year partnership between Ukraine and the UK".
It will also include £18m for humanitarian aid, help fortifying Ukraine's energy infrastructure and more funding for online English language training.
Armed Forces Minister James Heappey told OceanNewsUK the drones will give Ukraine an advantage over the coming years.
He said: "They are drones that are being developed at pace, learning all of the lessons from what we've seen in Ukraine over the last two years."
He added the funding shows the UK is maintaining its leadership position in Europe as the second-biggest donor to Ukraine.
During his visit the prime minister met with Mr Zelensky at the Presidential Palace and also met emergency services and residents of Kyiv.
Mr Sunak, who last visited Ukraine 15 months ago in November 2022, said: "I am here today with one message: the UK will also not falter. We will stand with Ukraine in their darkest hours and in better times to come."
"The UK is already one of Ukraine's closest partners, because we recognise their security is our security," he added.
"Today we are going further, increasing our military aid, delivering thousands of cutting-edge drones and signing a historic new security agreement to provide Ukraine with the assurances it needs for the long term.
"For two years, Ukraine has fought with great courage to repel a brutal Russian invasion. They are still fighting, unfaltering in their determination to defend their country and defend the principles of freedom and democracy."
When asked by OceanNewsUK if the UK should have instead announced a multi-year package of funding, the prime minister pointed to the UK being the first ally to sign a long-term security agreement with Ukraine.
"It sends a strong signal to Putin and others that we are here to stay," he said. "We are here to support Ukraine in the long term."
Mr Zelensky hailed Mr Sunak's "personal leadership" and said the agreement would lay "the groundwork for our further work with our other partners".
The UK-Ukraine agreement comprises a raft of bilateral assurances of military and economic support, designed to deter Russian aggression before Ukraine joins Nato.
Officials said the UK is the first G7 country to sign such a pact, which all seven leading industrial nations promised to do at the Nato summit in Vilnius last year.
The UK commitment comes after months of pressure from MPs, who argued the government should have given Ukraine greater clarity much earlier so it could develop its military planning.
It also comes as both the United States and the European Union struggle to agree their own packages of support, with Ukraine in desperate need of more shells and missiles.
Mr Sunak said he was confident allies "will come together to continue to support Ukraine".
In the US, Republicans are blocking a $60bn (£47bn) package of support for Ukraine in an attempt to increase spending on tackling migration along America's southern border.
And in the EU, Hungary's prime minister, Viktor Orban, has blocked a €50bn (£43bn) support package for Ukraine in a funding row with Brussels.
