Sadiq Khan reveals his 'ultimate goal' is to reverse Brexit as he declares: 'We need Europe!'

In the meantime, he called for Britain to rejoin the single market and customs union - flying directly in the face of his party.

Earlier this week, Downing Street ruled out rejoining the customs union after Valdis Dombrovskis, the European commissioner for economy and productivity, said the EU would be open to the idea.

SIr Sadiq said told The Observer: "Brexit was the biggest act of economic self-harm any country’s ever done."

"My ultimate goal is for us to rejoin and I think it will happen in my lifetime. I definitely want us to be part of a customs union.

"I’m somebody who’s an advocate for us being part of a single market. I think we can negotiate more now, because of how geopolitics has changed, than we could in 2016 or 2019.

"Europe needs us and we need Europe.”

Sir Sadiq has broken with party lines over rejoining the EU before.

In 2022, he said the "quickest way to get growth" would be to join the single market.

In August 2024, just one month into the new Labour Government, he called for the Brexit debate to be reopened.

He said: "In the medium to long term, there will need to be a conversation about whether we have a better future inside the EU or outside of it."

Sir Sadiq engages internationally with leaders of other major cities through the C40 Cities Climate Leadership Group.

The international group of 96 cities was founded through efforts by ex-London Mayor Ken Livingstone in 2005.

And the incumbent Mayor has been co-chairman of the group, which focuses on fighting the climate crisis and reducing emissions, since 2023.

Sir Sadiq won a record third term in City Hall in 2024 and has said he intends to run again in 2028.

Sir Keir Starmer has recently flirted with edging closer to the EU through defence programmes, but has been steadfast on not rejoining the EU.

However, potential leadership rival and Health Secretary Wes Streeting, and Deputy Prime Minister David Lammy, both have refused to rule out rejoining.

Business Secretary Peter Kyle said last month at the World Economic Forum that it would be "crazy" not to consider the idea of a customs union.

That came than 24 hours after he said Britain would be "foolish" to pursue one.