With a landmark post-Brexit EU-UK due summit next month, both sides are seeking to improve ties amid the global turmoil ushered in by US President Donald Trump.

However Starmer faces a tricky balancing act, as he is also looking to reach out to the Trump administration and secure a favourable trade deal with the United States.

UK and EU officials have been hoping a defence and security pact will be the highlight of the May 19 meeting, as Trump's return to the White House casts doubt on the United States' commitment to NATO and European security.

European Commission President von der Leyen said Thursday's talks could lead to the UK joining a European defence programme.

"We will discuss work on a strategic security and defence partnership agreement, which might pave the way then to a joint procurement and UK participation in our SAFE programme," she said, referring to a 150 billion euro ($170 billion) joint EU fund allowing countries to buy missiles, artillery, drones, ammunition and other equipment.

Britain needs to sign a defence pact to be included in the fund, which could benefit UK companies including BAE Systems and Rolls Royce.

- Post-Brexit 'reset' -

The London summit in May is also meant to turn the page on years of post-Brexit strife, as part of a "reset" in relations promised by Starmer.

Welcoming von der Leyen to Downing Street after the pair attended a global energy security summit in London, Starmer said: "In a world which seems increasingly unstable with an uncertain future, it is so good that we are working so closely together on so many issues."

"I'm really pleased that we've committed to a reset of our relationship, a really important relationship, which I think will be of huge benefit to both of us," he said.

The summit is set to be a key milestone in the Labour leader's pursuit of closer relations with the bloc following Britain's bitter 2020 departure under the previous Conservative government.

However Starmer has laid out strict red lines, while the EU has its own demands, raising questions about what exactly the talks can achieve.

The UK prime minister has vowed Britain will not return to the single market or customs union and has ruled out signing up to freedom of movement.

Some EU countries, led by France, have been pushing to link progress on defence agreements to successful negotiations in other areas, with fishing a major sticking point.