A security deal between the UK and Ukraine has been signed by Rishi Sunak and Volodymyr Zelensky during a surprise visit by the Prime Minister to Kyiv.
Britain will significantly boost Ukraine’s military capability, it was announced on Friday.
Mr Sunak pledged that the UK “will not falter” in its commitment to the country.
The PM said the Government would provide £2.5 billion in military aid for the fight against Russia over the coming year.
It will fund long-range missiles, air defence, artillery ammunition and maritime security.
About £200 million will be spent on a push to procure and produce thousands of military drones, many made in the UK. It is expected to be the largest delivery of drones to Ukraine from any nation.
Mr Zelensky described it as an “unprecedented security agreement” and thanked Mr Sunak for his "personal leadership" after the pair signed the accord.
The PM was awarded with the Ukrainian order of freedom honour recognising his "outstanding personal contribution into strengthening Ukrainian-British interstate co-operation, supporting state sovereignty and territorial integrity of Ukraine".
Speaking at a press conference alongside Mr Sunak, the Ukrainian President said the UK was laying "the groundwork for our further work with our other partners".
The PM is in Kyiv for just one day to sign the new UK-Ukraine Agreement on Security Cooperation.
Mr Sunak said: "This is the first in a series of new bilateral security assurances promised to Ukraine by 30 countries at last year's (Nato) Vilnius Summit.
"And it says that if Russia ever invades Ukraine again, the UK will come to your aid with swift and sustained security assurance."
Armed Forces minister James Heappey said the provisions intend to “really accelerate drone technologies” and show Vladimir Putin that there is “no loss in patience and commitment” from allies of Ukraine.
Mr Zelensky has been pressing allies in the West to provide more support to fight back against Russian aggression, amid fears that interest in the war is flagging as attention turns to the situation in the Middle East.
The crisis in Gaza as Israel continues to bombard the Palestinian territory has also turned global attention away from the battle against Putin.
Mr Sunak stressed the UK’s continued backing for Kyiv just hours after British and US forces launched strikes against military facilities used by Houthi rebels in Yemen.
Ahead of the meeting between the two leaders, he said: “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.
"I am here today with one message: the UK will also not falter. We will stand with Ukraine, in their darkest hours and in the better times to come.”
The PM made his first visit to Ukraine in November 2022, weeks after entering Number 10.
The UK has been among the most vocal backers of Ukraine, with Mr Zelensky visiting London early last year in a historic trip.
Mr Zelensky has argued that Ukraine particularly needs air defence systems to fend off Russian aerial barrages.
More than 500 drones and missiles were fired between December 29 and January 2, according to officials in Kyiv.
The Ukrainian leader began the year with a visit to several Baltic states in a bid to drum up support.
Mr Sunak said that the UK recognises that Ukrainian "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."
Labour leader Sir Keir Starmer also stressed it was "really important" that politicians remain focused and united on supporting Ukraine.
"We have had cross-party support for Ukraine for a very long time and we need to maintain that in the face of Russian aggression that has been going on for a long time now," he said during a visit to a dental school in Bury.
"So I'm supportive of what the Prime Minister is saying today and we will remain united across our political parties in defence of Ukraine against that aggression from Putin."
