The Prime Minister and the Ukrainian president met at Chequers for “substantive negotiations” over military aid ahead of a major counter-offensive from Kyiv against Russian forces.
Mr Sunak confirmed the provision of hundreds of air defence missiles and further unmanned aerial systems, including attack drones which have a range of over 125 miles.
Downing Street said the weapons were “critical in helping the country defend against the relentless bombardment of their national infrastructure” by Vladimir Putin’s army.
Mr Zelensky also repeated his plea for western countries to supply F-16 fighter jets to bolster his country’s defence, saying the issue had been discussed with Mr Sunak during their meeting.
New jets were a “very important topic for us, because we can't control the sky”, he said.
“We spoke about it and I see that in the closest time you will hear some, I think, very important decisions, but we have to work a little bit more on it.”
Mr Sunak said: “This is a crucial moment in Ukraine’s resistance to a terrible war of aggression they did not choose or provoke. They need the sustained support of the international community to defend against the barrage of unrelenting and indiscriminate attacks that have been their daily reality for over a year. We must not let them down.”
He compared Mr Zelensky’s wartime leadership to Winston Churchill as he welcomed the Ukrainian president to his country residence. Mr Sunak added: “You are actually the first foreign leader I have had the privilege of welcoming here as Prime Minister and there’s a lot of great history here.
“In fact this room that we are standing in, Winston Churchill made many of his famous speeches in World War Two from this room. And the same way today, your leadership, your country’s bravery and fortitude are an inspiration to us all.”
Mr Zelensky thanked the UK Government, the King and the British people for their support.
“We are thankful from all our hearts, from Ukrainians, from our soldiers, we are thankful,” he said.
The president added that Monday’s talks would cover “very important issues [and] urgent support” for his country.
The meeting comes after Britain last week agreed to provide Storm Shadow precision missiles to Ukraine — the first long-range cruise missile in the country’s arsenal — on top of the £2.3 billion worth of military support the UK pledged last year.
The Government also provided more details of the flying school for Ukrainian fighter jet pilots set to begin in Britain this summer.
The elementary flying phase is for cohorts of Ukrainian pilots to learn basic training and goes “hand in hand” with UK efforts to work with other countries on providing F16 jets, Downing Street said. The Prime Minister’s official spokesman confirmed that the UK has no plans to send fighter jets of its own to Ukraine.
Monday’s visit follows Mr Zelensky’s trip to London in February, when he met King Charles III and addressed MPs in Westminster Hall. It is also the president’s latest stop on a tour of European capitals as he seeks to bolster support for a Ukrainian counter-offensive.
He met Emmanuel Macron in Paris last night with the French president pledging additional military aid, including light tanks, armoured vehicles and soldier training.
It followed discussions in Germany over the weekend with Chancellor Olaf Scholz, who promised an aid package including 30 Leopard and 20 Marder tanks.
Mr Zelensky also picked up the prestigious Charlemagne Prize for European unity in Aachen, telling the audience that “peace was our primary duty to our countries and to Europe”.
Chair of the Defence Select Committee Tobias Ellwood said the tour was an “important reminder” that Ukraine is “fighting this European war for us”.
He told TalkTV: “He’s absolutely grateful for the support that he’s received but having to go around those capital cities in Europe to say, ‘let’s ensure that we don’t suffer from Ukraine fatigue, that we don’t lose sight of the fact that this is going to require not just one major counter attack, but a series of counter attacks to flush Russia out of the country’.
“If we don’t do that Russia will press on and do what it’s doing in Ukraine in other parts of Eastern Europe. This is attack on Western democracy. This is an attack on Europe and we are doing our part.”
The visit also comes ahead of the Council of Europe Summit in Iceland, which Mr Suank is due to attend and Mr Zelensky will address virtually, and the G7 Summit in Japan. Mr Sunak will use the gatherings to push for sustained support for Ukraine in the fight against Putin’s army, No 10 said.
In a tweet, Mr Zelensky said: “The UK is a leader when it comes to expanding our capabilities on the ground and in the air. This cooperation will continue today. I will meet my friend Rishi. We will conduct substantive negotiations face-to-face and in delegations.”
Mr Zelensky is expected to attend the next major Nato summit in person, with Ukraine pushing for a timeline for it joining the alliance.
Colonel General Oleksandr Syrskyi, commander of Ukrainian ground forces, said the operation had shown his troops can move forward and counter the Russian forces in the eastern town.
“The advance of our troops along the Bakhmut direction — that is the first successful offensive operation in (Bakhmut’s) defence,” he was quoted as saying on Ukraine’s Media Military Centre Telegram messaging platform.
“The last few days have shown that we can move forward and destroy the enemy even in such extremely difficult conditions… The operation to defend Bakhmut continues. All necessary decisions for the defence have been made.”
Moscow admitted on Friday that Russian troops had retreated to “more favourable positions” near the Berkhivka reservoir northwest of Bakhmut.
Kyiv is expected to launch a major counteroffensive soon to try to retake Russian-occupied territory, but Ukrainian officials have indicated that the gains around Bakhmut do not signal that the broader counterattack has begun.
Deputy Defence Minister Hanna Maliar said on Monday that heavy fighting continued in and around Bakhmut, and that “everything” was difficult there.
“The Russians have not changed their goals. They are sending assault troops to the outskirts of Bakhmut,” she wrote on Telegram.