"We have just reached an agreement with Britain on security in all areas: on the ground, in the air, at sea, in cyberspace," Ukrainian public broadcaster Suspilne quotes Zelenskyy.
The agreement is designed to last until Ukraine joins NATO but can be extended if needed.
"If this [Ukraine's accession to NATO] happens before the expiration of this agreement, the security architecture we have created will actually be incorporated into the security system of the entire alliance," Zelenskyy added.
London will share intelligence with Kyiv, assist with national cybersecurity, military training, and develop mutual defense industry cooperation, according to a statement on the UK government’s website.
The deal also commits the UK to consult with Ukraine in case of another Russian invasion and to provide "swift and sustained" military assistance.
Zelenskyy added that if similar guarantees had been reached in 1991—including with the UK—Russia would never have dared to invade in the first place.
The agreement marked the first implementation of broad security arrangements reached at the 2023 NATO Vilnius summit between Ukraine, G7 nations, and other partners.
"Today we stand side by side as friends and allies; this is a signal to the world: Ukraine is not alone," the Ukrainian president said.
This is Sunak’s second trip to Ukraine, after he arrived in Kyiv on Nov. 19, 2022, shortly after replacing Liz Truss at Downing Street 10.