In short:
The UK and France will work on a peace plan with Ukraine to present to the US, as European leaders commit to maintaining support for the country for the rest of the war.
European Commission President Ursula von der Leyen says leaders agreed to "desperately re-arm" and to provide support to turn Ukraine into a "steel porcupine".
What's next?
A number of countries have indicated they wanted to be part of Europe's planned "coalition of the willing" to help Ukraine after the war, Mr Starmer said.
British Prime Minister Keir Starmer is warning Europe is "at a crossroads in history" as the continent's leaders pledge to re-arm their militaries and turn Ukraine into a "steel porcupine" in the face of Russian aggression.
Amid concern the US may be willing to end support for Ukraine, leaders of a number of European countries met in London on Sunday to work on a plan to end the conflict.
Ukraine peace plan updates: Follow our blog for the latest updates on the European leaders summit
The UK and France have now committed to developing a peace plan with Ukraine to end the Russian invasion, with Sir Keir saying he would not "be going down this road" if he did not feel he had American support.
Sir Keir also revealed a $3.2 billion package that would see the UK begin building air defence missiles in Northern Ireland to supply to Ukraine.
Volodymyr Zelenskyy met with King Charles after the summit of European leaders in London. (Reuters: Joe Giddens)
The summit, which was attended by Ukrainian President Volodymyr Zelenskyy, came just two days after he was involved in a heated meeting with US President Donald Trump and his Vice-President JD Vance in the Oval Office.
Mr Zelenskyy met with King Charles III at his estate in Sandringham on Sunday.
Speaking after the summit, Sir Keir said Russia could not be allowed to "dictate terms" of a peace deal with Ukraine.
"Our starting point must be to put Ukraine in the strongest possible position now so they can negotiate from a position of strength," he said.
"We are doubling down on our support."
Volodymyr Zelenskyy, Keir Starmer and Emmanuel Macron are committing to produce a plan to end the war in Ukraine. (Reuters: Justin Tallis)
Sir Keir said European leaders had agreed to keep military aid flowing to Ukraine and to maintain economic pressure on Russia, and wanted to be a part of the "coalition of the willing".
"In the event of a peace deal, we will keep boosting Ukraine's own defensive capabilities to deter any future invasion," he said.
He urged other European nations to also commit to putting peacekeepers in Ukraine once a peace deal is reached, warning "we have to learn from the mistakes of the past".
"The UK is prepared to back this with boots on the ground and planes in the air, together with others. Europe must do the heavy lifting."
Protests in support of Ukraine were held on the streets of Central London. (Reuters: Isabel Infantes)
US relationship 'still alive and kicking'
After the summit, European Commission President Ursula von der Leyen said security guarantees were crucial for turning a post-war Ukraine into a "steel porcupine that is indigestible for potential invaders".
She said European nations needed to "surge" their defence spending to "desperately re-arm Europe".
"We are ready together with [the United States] to defend democracy, to defend the principle of there is a rule of law and that you cannot invade your neighbour or bully your neighbour,"
she said.
In an interview with the BBC, Finnish President Alexander Stubb said the NATO alliance between Europe and the US was still "alive and kicking", but added the relationship with the United States was becoming more "transactional".
He said the scenes in the Oval Office on Friday were akin to conversations he had witnessed behind closed doors, but conceded the public spectacle was damaging to the West and benefited Russia.
The London summit of Europe's "coalition of the willing" have committed to rapidly increase defence spending. (Reuters: NTB/Javad Parsa)
Speaking to CNN, US National Security adviser Mike Waltz rejected allegations the Oval Office meeting between Mr Trump, Mr Vance and Mr Zelenskyy was an "ambush".
Instead, he said Mr Zelenskyy appeared not ready to "negotiate in good faith" on steps to end the war.
Speaking at the summit, Sir Keir said he did not accept that the US was an unreliable ally.
After the meeting, Mr Zelenskyy said he had left London with "Europe's clear support" and readiness to cooperate.
"There will be diplomacy for the sake of peace," he said in his nightly video address.
"And for the sake of us all being together — Ukraine, all of Europe and definitely, definitely America."
Russia scolds Europe's plans
As European leaders gathered in London, Russian Foreign Minister Sergei Lavrov praised Mr Trump's "commonsense" aim to end the war in Ukraine.
After clash in the Oval Office, what now for Ukraine?
Photo shows Volodymyr Zelenskyy boarding a Ukrainian government plane on stairs.
Russian media and government officials have reacted with glee to the Oval Office confrontation between Mr Trump and Mr Zelenskyy.
"Donald Trump is a pragmatist," Mr Lavrov told the Russian military newspaper Krasnaya Zvezda.
"His slogan is common sense. It means, as everyone can see, a shift to a different way of doing things."
Mr Lavrov also dismissed European ideas for sending in a contingent of peacekeepers to Ukraine to guarantee its post-war security.
He said Russia had no trust in Ukraine after the collapse of previous ceasefire agreements, which were designed to end a separatist war by Russian speakers in eastern Ukraine.
"Now they also want to prop [Mr Zelenskyy] up with their bayonets in the form of peacekeeping units. This will mean that the root causes will not disappear," Mr Lavrov said.
ABC/Reuters/AP