The British government says Starmer and Ukrainian President Volodymyr Zelenskiy will sign a "100-Year Partnership" treaty in Kyiv, covering areas including defence, science, energy and trade.
Starmer's unannounced visit on Thursday is his first trip to Ukraine since he took office in July.
Prime Minister Keir Starmer has previously hosted Ukrainian President Volodymyr Zelenskiy in London. (AP PHOTO)
He visited the country in 2023 when he was opposition leader, and has twice held talks with Zelenskiy in 10 Downing Street since becoming prime minister.
One of Ukraine's biggest military backers, the UK has pledged Stg12.8 billion ($A25 billion) in military and civilian aid to Ukraine since Russia's full-scale invasion three years ago, and has trained more than 50,000 Ukrainian troops on British soil.
Starmer is due to announce another Stg40 million for Ukraine's postwar recovery.
But the UK's role is dwarfed by that of the United States, and there is deep uncertainty over the fate of American support for Ukraine once Trump takes office on January 20.
The president-elect has baulked at the cost of US aid to Kyiv, says he wants to bring the war to a swift end and is planning to meet Russian President Vladimir Putin, for whom he has long expressed admiration.
Kyiv's allies have rushed to flood Ukraine with as much support as possible before Trump's inauguration, with the aim of putting Ukraine in the strongest position possible for any future negotiations to end the war.
Ukraine's allies have rushed to flood it with support to put Kyiv in a strong negotiating position. (AP PHOTO)
Zelenskiy has said that in any negotiation, Ukraine would need assurances about its future protection from its much bigger neighbour.
Britain says its 100-year pledge is part of that assurance, and will help ensure Ukraine is "never again vulnerable to the kind of brutality inflicted on it by Russia", which seized Crimea from Ukraine in 2014 and attempted a full-scale invasion in February 2022.
The deal commits the two sides to co-operate on defence - especially maritime security against Russian activity in the Batlic Sea, Black Sea and Sea of Azov - and on technology projects such as drones.
The treaty also includes a system to help track stolen Ukrainian grain exported by Russia from occupied parts of the country.
"Putin's ambition to wrench Ukraine away from its closest partners has been a monumental strategic failure," Starmer said before the visit.
"Instead, we are closer than ever, and this partnership will take that friendship to the next level."
Donald Trump has appeared to sympathise with Vladimir Putin's opposition to Ukraine being in NATO. (AP PHOTO)
Zelenskiy says he and Starmer would also discuss a plan proposed by French President Emmanuel Macron that would see troops from France and other Western countries stationed in Ukraine to oversee a ceasefire agreement.
Zelenskiy has said any such proposal should go alongside a timeline for Ukraine to join NATO.
The alliance's 32 member countries say Ukraine will join one day, but not until after the war.
Trump has appeared to sympathise with Putin's position that Ukraine should not be part of NATO.
As the grinding war nears the three-year mark, both Russia and Ukraine are pushing for battlefield gains before possible peace talks.
Ukraine has started a second offensive in Russia's Kursk region, where it is struggling to hang on to a chunk of territory it captured in 2024, and has stepped up drone and missile attacks on weapons sites and fuel depots inside Russia.
Moscow is slowly taking territory at the cost of high casualties, along the 1000km front line in eastern Ukraine and launching intense barrages at Ukraine's energy system, seeking to deprive Ukrainians of heat and light in the depths of winter.
A major Russian ballistic and cruise missile attack on regions across Ukraine on Wednesday, and compelling authorities to shut down the power grid in some areas.
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