After a heated row with Trump, who threatened to stop support for Ukraine after accusing Zelenskiy of being ungrateful for US aid, the Ukrainian leader flew to London on Saturday to be greeted on Downing Street with a long, warm hug from Starmer.
Zelenskiy will receive a pledge of support from Starmer and other European leaders, who face the stark question of whether they can take over the lead in providing Kyiv with weapons and finance before any peace talks begin.
Lacking the weaponry and depth of ammunition stocks of the US, European leaders have so far offered expressions of support after the Zelenskiy row, with Germany calling for the release of three billion euros ($A5 billion) for Ukraine.
UK Chancellor Rachel Reeves and Ukraine's Finance Minister Sergii Marchenko signed an agreement to provide a Stg 2.26 billion ($A4.58 billion) loan to Ukraine, according to a statement from the Treasury on Saturday.
On Sunday, Starmer will hope they will offer more concrete means to support Ukraine and try to revive a possible peace deal with Russia by convincing Trump Europe can step up to defend itself. Some leaders might also encourage Zelenskiy to go back into talks with the US leader.
"Three years on from Russia's brutal invasion of Ukraine, we are at a turning point," Starmer said in a statement, offering his "unwavering support for Ukraine" by doubling down on providing capacity, training and aid to Kyiv.
"In partnership with our allies, we must intensify our preparations for the European element of security guarantees alongside continued discussions with the United States."
At a meeting that has taken on added significance, Starmer will start on Sunday by holding talks with Italian Prime Minister Giorgia Meloni after welcoming Zelenskiy on Saturday with a clear message of support for the shaken leader.
They will then be joined by French President Emmanuel Macron, the EU's Ursula von der Leyen, NATO Secretary General Mark Rutte and leaders from Germany, Denmark, Netherlands, Norway, Poland, Spain, Canada, Finland, Sweden, the Czech Republic and Romania. Turkey's foreign minister will also attend.
Zelenskiy's row with Trump on Friday ended a week when Europe had appeared to be in a better position in its drive to encourage Trump to continue to offer support to Ukraine after cordial visits to Washington by Macron and Starmer.
Both had pressed the US leader to offer what is known as "a backstop" to a potential European peacekeeping force in the event of a deal between Kyiv and Moscow. While they had failed to secure a promise from Trump, he had not totally rejected the idea.
But the rest of Europe is a long way from falling in behind the French and British leaders' plans to safeguard a peace deal, and Starmer will hope to take on a leadership role by suggesting Britain become a bridge between Europe and the United States.
Sunday's meeting is a boost for Starmer, whose team felt his meeting with Trump last week had gone better than expected with Trump praising the British leader and even suggesting Ukraine could regain some lost territory in peace talks.
Friday's disastrous meeting poisoned that mood, and Sunday's summit and an extraordinary EU one in Brussels next week will prove crucial to establishing whether European leaders can offer Ukraine something concrete and repair the damage done.
Italian Prime Minister Giorgia Meloni has also called for an emergency summit involving the US, European nations and allies to discuss how "to deal with the great challenges of today, starting with Ukraine".
On Sunday, European leaders are expected again to press for peace talks to include Ukraine, to strengthen Kyiv's position and to secure the necessary security guarantees to ensure a long-lasting peace and to deter any future Russian attack.
That would mean persuading Washington to offer a back stop, possibly in the form of air cover, intelligence and surveillance and a greater as yet unspecified threat if Putin again sought to take more territory.
Starmer will update other leaders on his meeting with Trump, when the US president praised his efforts to increase defence spending and offer to deploy peacekeeping troops.
For now, Starmer wants to lower the temperature after the row in Trump's Oval Office.
Some leaders will also urge Zelenskiy to try to rebuild bridges with Trump. NATO's Rutte and Polish President Andrzej Duda have urged the Ukrainian leader to find a way to restore the relationship.
with PA and AP
