The UK will provide £2.5bn of military aid to Ukraine over the coming year, Rishi Sunak has said - Britain's largest annual commitment since Russia's invasion.
The PM made the announcement during a rare visit to the country, where he will also sign a new agreement supporting its long-term security.
Officials said the package will provide Ukraine with long-range missiles, air defence and artillery shells.
Some £200m will be spent on drones, most of which will be UK-made.
Officials said the military package - for the next financial year beginning in April - would result in the largest delivery of drones 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.
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."
He added: "The UK is already one of Ukraine's closest partners, because we recognise their security is our security.
"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."
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.
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.
The UK-Ukraine agreement - being signed today - 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.
