Labour should include a pledge to rejoin the European Union in the party’s next general election manifesto, Sadiq Khan has declared.

In comments which will be seen as a direct challenge to Keir Starmer, the London mayor said it was time for the party to “be bold” in its offer to voters.

In an interview with Italian newspaper La Repubblica, Khan repeated his call for Britain to rejoin the EU customs union and single market in this parliament – a move already ruled out by the prime minister.

But he said Labour should go even further by pledging to take the UK back into the EU if it wins the next election, which is expected in 2029.

Khan said: “I’m quite clear. On the ballot paper of the next general election is a vote for Labour, a vote to rejoin the European Union, and we should be unequivocal about the benefits of the European Union because we’ve now seen the alternative.

“We’ve now seen what happens when you’re outside the European Union: less investment in the UK, less exports to the European Union.”

Asked in 2024 whether he could foresee the UK rejoining the single market or customs union in his lifetime, Starmer said: “No. I don’t think that that is going to happen. I’ve been really clear about not rejoining the EU, the single market or the customs union.”

However, in his interview, Khan said it was “inevitable” that the UK would once again become an EU member state now that voters have seen the damage done by Brexit.

He said: “We’ve now seen what happens when you’re outside the European Union: less investment in the UK, less exports to the European Union.

“But let me speak from experience. You know, since 2019, and it breaks my heart, I’ve seen Londoners who are EU nationals leaving London.

“We had in 2019 more than 840,000 EU Londoners working in London. That’s gone down to now 700,000, that means 140,000 Londoners have left London, and the two biggest sectors they’ve left concerned construction and hospitality.

“And these are Romanians, Polish, Italian, French, Irish Londoners, who’ve left their friends. They’ve left their neighbours. As a consequence, we’ve suffered economically, in construction, hospitality, but also we’ve suffered socially and culturally as well.”