LONDON: Seeking to shore up support more than three years into Russia's invasion, Ukrainian President Volodymyr Zelenskyy met UK Prime Minister Keir Starmer during a surprise visit to London on Monday (Jun 23), cementing a military co-production deal.

Zelenskyy met with Starmer at his Downing Street home, after earlier visiting King Charles III at Windsor Castle.

The trip comes on the eve of a NATO summit in The Hague, which Zelenskyy is due to attend.

Addressing Ukrainian military personnel undergoing training in the UK, Starmer said the pair had had "an excellent bilateral meeting" and agreed an "industrial military co-production agreement", which he called "a massive step forward in the contribution that we can continue to make".

Zelenskyy, speaking beside Starmer, insisted the deal "will be very strong and will transform both nations", although no details were released.

After the meeting, Starmer said it was "really a privilege, a pleasure" to welcome Zelenskyy, calling him "a regular now at Downing Street".

Starmer told the Ukrainian troops it was "really humbling" to see their "level of professionalism, commitment and bravery".

More than 50,000 troops have now been trained as part of the international partnership.

Zelenskyy said the scheme had helped "strengthen our army" and enabled Ukraine to "survive and fight".

The UK has been one of Ukraine's staunchest supporters since Russia invaded in early 2022, levelling rounds of sanctions against Moscow and supplying multiple packages of military aid.

Crucial summit

Starmer promised the support would continue "for the rest of the conflict" and help put Ukraine in "the strongest possible position" to negotiate a ceasefire.

Zelenskyy said his country was "very thankful to the UK... for such big support of Ukraine from the very beginning of this war".

The Ukrainian leader earlier travelled to Windsor Castle, where he "visited The King... and remained to luncheon", Buckingham Palace said.

Zelenskyy is expected at the NATO summit in The Hague on Tuesday and Wednesday, where Ukraine's allies will work "to ensure that Ukraine is in the best possible position as we go into the next stage of this conflict", according to Starmer.

NATO allies are poised to take a "quantum leap" by hiking defence spending to counter the threat of Russia, Secretary General Mark Rutte said on the eve of the two-day summit.

The alliance's 32 members will pledge to boost defence expenditure to five percent of gross domestic product, a key demand of President Donald Trump, who has long grumbled that the US pays too much for NATO.

NATO's members have thrashed out a compromise deal to dedicate at least 3.5 percent of GDP to core military needs by 2035, and 1.5 percent to broader security-related items like cyber-security and infrastructure.

"The defence investment plan that allies will agree in The Hague introduces a new baseline, five percent of GDP to be invested in defence," Rutte told reporters at a pre-summit news conference.

"This is a quantum leap that is ambitious, historic and fundamental to securing our future."