Sir Keir Starmer is set to discuss tightening sanctions on Russia as he meets Volodymyr Zelensky and other European leaders in Tirana on Friday.
The Prime Minister confirmed Mr Zelensky would attend the European Political Community summit in the Albanian capital to discuss both support for Ukraine and efforts to secure a ceasefire with Russia.
The summit comes as Ukrainian and Russian negotiators meet face-to-face for the first time in Istanbul.
But a breakthrough is considered unlikely after Mr Putin refused to turn up for talks on Thursday, sending a low-level delegation instead.
Speaking to reporters outside the summit in Tirana, Sir Keir said this was “more evidence that Putin is not serious about peace”.
He added: “He’s been dragging his heels and I think it’s really important therefore that we have absolute unity with our allies.
“We’ll be working on that again today to be clear that there must be a ceasefire, but also to be clear that should there not be a ceasefire then we will act together in relation to sanctions.”
After entering the summit venue, Sir Keir spoke with some of his counterparts including Mr Zelensky and French president Emmanuel Macron, who along with the Prime Minister has led efforts to create a “coalition of the willing” to provide peacekeeping forces for Ukraine.
More than 40 leaders have arrived in the Albanian capital for talks, which are expected to focus on intensifying sanctions on Russia if the Kremlin does not agree to a ceasefire, with Russian energy likely to be a target.
Moscow has so far resisted calls for an unconditional 30-day ceasefire backed by both Kyiv and Washington.
Speaking at the summit, Sir Keir said allies must be willing to “follow through” on their “ultimatum” to Vladimir Putin if Russia continues to refuse to properly engage with peace talks.
The Prime Minister insisted it was an important moment to “together, and supported by President Trump,” push the argument for an unconditional ceasefire.
“In issuing that ultimatum, we must be prepared to follow through, because if Russia won’t come to the negotiating table, Putin must pay the price,” he told European leaders.
“In this time of great uncertainty and volatility, the UK will not respond by turning inwards, but by taking its place on the world stage, strengthening our alliances, supporting freedom and peace in Europe.”
Also in Tirana, Nato secretary-general Mark Rutte told reporters that Ukraine would be “number one on the agenda” for the summit, adding that Mr Putin had made “a big mistake” by sending a low-level delegation to Istanbul.
He said: “Ukraine clearly wants to play ball. They are very serious about this.”
As well as discussing support for Ukraine, leaders in Tirana are likely to focus on wider European defence issues and migration.
Sir Keir himself is expected to hold meetings with some of his European counterparts in the margins of the summit as he prepares for a meeting with the EU in London on Monday.
Reports suggest a deal on British access to a major EU defence fund could be on the table on Monday, but arguments over fishing rights and a youth mobility scheme may provide stumbling blocks to an agreement.
Kaja Kallas, the EU’s high representative for foreign affairs, said work was progressing on a defence deal, but added: “We’re not there yet.”
The Prime Minister insisted he was “positive” ahead of Monday, while European Commission president Ursula von der Leyen said she was “looking forward” to the meeting.
Sir Keir has repeatedly declined to give a “running commentary” on the talks or engage in what he described as “megaphone diplomacy” with the EU.
But he told reporters in Tirana on Thursday he was “confident” of making “really good progress into Monday”.
He was later seen speaking to Ms von der Leyen inside the summit venue.
