British Prime Minister Keir Starmer said 2026 could mark meaningful progress toward ending the war in Ukraine.
Starmer added that he believes they were "much further advanced" than they "were ever before" on whether the United States will stand ready to give security guarantees to the war-torn nation.
Russian President Vladimir Putin may pause the invasion but he will be "back at it" without strong guarantees in place in the event of a peace deal, he told the BBC's "Sunday With Laura Kuenssberg."
This comes ahead of a meeting with the Coalition of the Willing in Paris on Tuesday that will include Volodymyr Zelenskyy.
The Ukrainian president has said there will be talks with U.S. representatives during the trip.
Starmer told the BBC programme that there has been "serious progress on the American [plan] and on the Coalition of Willing to a point now where one of the discussions we'll have on Tuesday is, how do we integrate what the Americans are putting on the table with what the coalition of the willing countries [are]?"
Asked whether that meant joining forces, the prime minister said: "How do we integrate that? We've had military planning. We've got military plans in place, so the question we're now on is, how do we integrate those? A security guarantee that's a long way further forward than we were when we set up the coalition the willing."
He added: "Without being over optimistic, because these things can move around, 2026 seems to me the year in which we could make significant progress towards peace in Ukraine, and what a difference that will make to the United Kingdom."
Issues at home are "directly linked" to the war in Ukraine, he said.
Starmer also told Kuenssberg that he was "never going to choose between the US and Europe," but said Europe had become "over-reliant" on America.
"It's time that, as European leaders, we stepped up on defence and security and intelligence, not just on the amount we spend, but how we collaborate and co-operate".
He added: "It has shone a torch, in my view on the question of whether Europe has done enough in its own defence and security, not just on Ukraine, but more generally, but we've been found wanting."
The future of Donetsk will be another "sticking point," and "I'm really, really clear that issues of territory have to be for Ukraine," he added.
Zelensky has continued to resist Russian demands that Ukraine gives up Luhansk and Donetsk, both regions in the east.
The coalition of the willing is made up of around 30 countries who support Kyiv's effort to end the war with Russia on acceptable terms.
