Sir Keir Starmer is to travel to Brussels to attend a summit of European Union leaders – the first time a British prime minister has done so since Brexit.
Starmer is to cross the Channel for talks that will focus on defence and security co-operation and will also meet Nato secretary general Mark Rutte.
The trip is part of what he calls a "reset" between the UK and the European Union.
The UK will not rejoin the EU's single market or customs union or sign up to freedom of movement, the government has promised.
But ministers do want what they see as a better relationship on defence and security, crime and trade.
They look to sort that out by spring and finalize at a UK-EU summit, perhaps in April or May.
Leaders of the European Union's 27 member states are convening for what is billed as an "informal retreat" at the Palais d'Egmont in Brussels - a 16th century palace at the heart of the Belgian capital.
The backdrop is clear: the ongoing war in Ukraine and the return of Donald Trump to the White House.
The EU recognizes it has to take more responsibility for its own defense – a key and recurring demand of President Trump, as he threatens the bloc with import taxes or tariffs.
This is what the prime minister said: "President Trump has threatened more sanctions on Russia, and it's clear that's got Putin rattled. We know he is worried about the state of the Russian economy.
"I'm here to work with our European partners on keeping up the pressure, targeting the energy revenues and the companies supplying his missile factories to crush Putin's war machine.".
"Because ultimately, alongside our military support, that is what will bring peace closer."
But Trump's cryptic, threatening and yet tantalising comments overnight on trade with the UK, where he said Britain was "out of line" on trade things "can be worked out", neatly illustrate a dilemma Whitehall's been conscious of ever since the president's re-election.
That might just tread on Washington's toes. Overdo the overtures in the Oval Office and the EU will be miffed. After Monday's visit to Brussels, the prime minister is expected in the US in the next few weeks.
The UK is also considering closer ties with the EU over serious and organized crime, but importantly, over trade.
Discussion is being had on allowing food and animal products to be traded more freely, cooperation on energy with a possible tie-up between the UK and the EU's emission trading schemes, mutual recognition of professional qualifications, and the ability for touring musicians to travel more easily.
Many in the EU are interested in a youth mobility scheme where young people from the UK and the EU will be able to travel much more easily.
However, such a scheme will sound to some rather like freedom of movement, albeit for a narrow chunk of the population and so may be a hard sell for the UK politically.
It is also likely, given the respective sizes of the EU and the UK, that more young people from the EU would come to the UK than vice versa.
Ministers have rejected the idea so far.
The EU has also floated the UK joining what is known as the Pan-Euro-Mediterranean Convention.
The Convention is not exactly a customs union, but it does eliminate customs on the parts that are put into the product sourced from the countries that are signed up to it.
It can be noted that the UK has not ruled out signing up to this and it is not seen by government as a breach of its red lines.
Some sectors, such as the car industry, with its 'just in time' supply chains, would probably welcome such a move but other manufacturers would be open to more competition.
Leading the technical negotiations for the UK is Michael Ellam, who worked in Downing Street when Gordon Brown was prime minister.
Ellam has been given the task to oversee what is termed as the "EU Relations Secretariat" within the Cabinet Office. The prime minister had set this up shortly after the general election to lead on his planned "reset" with the EU.
As the negotiations continue, he is well aware that political pressure from both sides of the argument will be on him.
Conservative leader Kemi Badenoch has claimed "the Labour government are trying to reopen the divisions of the past and edge us back into the EU".
Liberal Democrats leader Sir Ed Davey has said the government should be negotiating to re-join the customs union.
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