Rishi Sunak has visited Ukraine for the first time since becoming prime minister, meeting President Volodymyr Zelenskyy in Kyiv.
The package comprises 125 anti-aircraft guns and technology to counter Iranian-supplied drones, including dozens of radars intended to boost anti-drone electronic warfare capability.
The UK will also increase training for Ukraine's armed forces, sending expert army medics and engineers to the region to offer specialised support.
During his visit, Mr Sunak laid flowers at a memorial for the war dead, lit a candle at a memorial for victims of the Holodomor famine, and met Ukrainian first responders including emergency personnel at a fire station.
A No 10 spokesperson confirmed the prime minister's trip, saying: "The prime minister is in Ukraine today for his first visit to Kyiv to meet President Zelenskyy and confirm continued UK support."
Mr Sunak tweeted: "Britain knows what it means to fight for freedom. We are with you all the way."
President Zelenskyy shared a short video on his official Telegram social media feed showing the UK PM getting out of a car and walking with him along a red carpet into the Mariinskyia Palace.
The two leaders appear to make a comment about the cold weather as they shake hands, with small flakes of snow falling all around.
Wearing a flag pin showing the British and Ukrainian flags side-by-side, Mr Sunak is shown in a one-to-one meeting with the Ukrainian president, as well as a larger roundtable style meeting with military leaders.
A message from the Ukrainian leader accompanying the video said: "Since the first days of the war, Ukraine and the UK have been the strongest of allies.
"During today's meeting, we discussed the most important issues both for our countries and for global security.
"Together we are stronger and we will achieve the desired results."
Mr Zelenskyy also tweeted his thanks to Mr Sunak, writing: "With friends like you by our side, we are confident in our victory. Both of our nations know what it means to stand up for freedom."
Following his visit, Mr Sunak said in a statement: "I am proud of how the UK stood with Ukraine from the very beginning. And I am here today to say the UK and our allies will continue to stand with Ukraine, as it fights to end this barbarous war and deliver a just peace.
"While Ukraine's armed forces succeed in pushing back Russian forces on the ground, civilians are being brutally bombarded from the air. We are today providing new air defence, including anti-aircraft guns, radar and anti-drone equipment, and stepping up humanitarian support for the cold, hard winter ahead.
"It is deeply humbling to be in Kyiv today and to have the opportunity to meet those who are doing so much, and paying so high a price."
Mr Sunak follows in the footsteps of former prime ministers Boris Johnson and Liz Truss, pledging the UK's ongoing support for Ukraine amid the Russian invasion.
The prime minister also used his appearance at the G20 this week to join with allies and other Western leaders to condemn Vladimir Putin's invasion.
The visit comes after a missile hit a Polish village close to the Ukrainian border, killing two people, and initially sparking concerns Russia had struck a NATO member state.
Tensions have since been eased by the likely attribution of the hit to Ukrainian defence forces.
In the last week, Ukrainian forces say Russia has rained down more than 148 missile strikes on critical infrastructure, leaving approximately 10 million people without power.