London Mayor Sadiq Khan has told European Union diplomats that Brexit was "a mistake" and urged stronger ties between the United Kingdom and EU to counter US President Donald Trump's tariff threats.

Addressing the EU ambassador and the UK ambassadors to the 27 member states at a meeting he hosted on Tuesday, Khan said the UK's departure from the union "continues to have a negative impact" on the country and its capital city and pledged to push for closer alignment with the bloc, reported The Guardian newspaper.

While UK Prime Minister Keir Starmer's government seeks closer EU ties five years after Brexit, it has rejected rejoining the single market or allowing freedom of movement.

Khan told the ambassadors that he backed stronger relations and supported a proposed youth mobility program between the UK and EU.

"This would help to aid economic growth across Europe, but also give young Londoners and EU citizens important life experiences — like the opportunity to work abroad and learn more about our respective languages and cultures," he said.

"As part of this, I'm keen for us to look at how we can make it easier for schoolchildren from the EU to visit the UK and learn more about our shared ties and history."

Starmer has consistently rejected proposals for a youth mobility initiative with the EU, maintaining his stance against any policy resembling free movement.

Relationship reset

A government spokesperson said: "We are committed to resetting the relationship with the EU to improve the British people's security, safety and prosperity. But we have been clear there will be no return to freedom of movement, the customs union or the single market."

As part of his Brexit reset strategy, Starmer has begun rebuilding European partnerships, signing defense agreements with Germany and Estonia in October that focus on security cooperation and investment.

Khan declared "wholehearted support" for the prime minister's plan to reset relations with Europe and called for stronger UK-EU cooperation amid Trump's trade threats, according to The Independent website.

"I remain passionate about growing and improving our relationship across every area possible, and I believe this is essential if we're to effectively tackle a host of shared challenges relating to trade, our economies, security, the environment and the rise of an intolerant and anti-democratic populism," he said.

"Indeed, at a moment when we see trade wars and tariffs posing a real threat to international affairs, I'm convinced that we should be looking at what more we can do to strengthen our relationship as a counterweight to these trends."