Starmer said during a meeting with German Chancellor Olaf Scholz on Sunday that he was "very much looking forward" to attending Monday's informal EU leaders retreat in Belgium.

While there, the prime minister will discuss plans for a UK-EU defence and security partnership "in order to tackle the generational threats we all face," according to a press release sent out by his Downing Street office late Sunday.

He will also call on European countries to "continue bearing down on Putin" and "step up and shoulder more of the burden in order to keep Europe safe against Russia's increasing campaign of sabotage and destruction on our continent".

In the talks with Scholz, Starmer stressed that it was important to ensure Ukraine "was in the strongest possible position in the coming months", so that any peace deal to end its war with Ukraine "could be achieved through strength."

"President Trump has threatened more sanctions on Russia and it's clear that's got Putin rattled," Starmer said ahead of the trip.

"We know that he's worried about the state of the Russian economy."

By keeping up sanction pressure on Russia's energy revenues and companies supplying its missile factories, the UK and its European partners could "crush Putin's war machine," he added.

Starmer will meet with NATO Secretary General Mark Rutte on Monday, before travelling to meet with the leaders of the 27 EU Member States at an informal meeting of the European Council.

The session is part of Starmer's "ongoing commitment to strengthen our partnership with the European Union," said Downing Street.

Resetting post-Brexit relations with the EU is a key foreign policy strategy for Starmer, who was a strong supporter of staying in the bloc during the 2016 referendum.