Sadiq Khan has stoked fears of a Labour plot to unravel Brexit by holding talks on Europe with Spain's socialist prime minister.
The London mayor held a 'bilat' with Pedro Sanchez in Madrid today, despite his office having no foreign policy role, citing the 100,000 Spaniards living in the city.
Allies of the mayor said that Mr Sanchez used the meeting to reiterate Spanish support for Britain rejoining the European Union.
A source said: 'Sadiq has said it before and he'll say it again — Brexit has done real damage to London and is costing Londoners, day in, day out, economically, socially and culturally.
'Hearing directly from Prime Minister Sánchez that Spain would welcome the UK back into the EU was incredibly powerful. Europe needs us and we need Europe.'
Both Prime Minister Keir Starmer and Chancellor Rachel Reeves have made clear their wish to forge closer ties with the bloc as they scramble to boost economic growth.
Labour is planning to introduce legislation that would allow ministers to sign the UK up to EU single market rules without a full vote in Parliament.
Posting an image of himself with Mr Sanchez on X, Mr Khan hailed their shared interest in 'progressive, internationalist politics', adding: 'At a time when too many leaders are looking the other way, it matters that we stand together.
'We discussed Gaza, Europe and the shared values that unite London and Spain.'
The London mayor held a 'bilat' with Pedro Sanchez in Madrid today, despite his office having no foreign policy role
Both Prime Minister Keir Starmer and Chancellor Rachel Reeves have made clear their wish to forge closer ties with the bloc as they scramble to boost economic growth
Yesterday Keir Starmer's top negotiator defended the Prime Minister's efforts to sign Britain back up to EU rules as part of his Brexit 'reset' plan.
Nick Thomas-Symonds, the EU relations minister, insisted 'alignment' with Brussels 'is not a dirty word' as he dismissed criticism of the UK moving back towards the EU.
The plans to ease 'dynamic alignment' with EU rules would happen using secondary legislation under so-called 'Henry VIII' powers.
A new bill, designed to bring a proposed UK-EU deal on food and drink into force, will mean - once an agreement is reached with the bloc - any further EU changes deemed in the national interest can be waved through without an official vote by MPs.
Brexiteers have slammed the plans as 'exactly what the country rejected' at the EU referendum almost a decade ago, while some Labour figures have urged Sir Keir to go further and campaign to fully rejoin the bloc.
Mr Thomas-Symonds acknowledged Sir Keir's Brexit 'reset' would be a 'central issue' ahead of the next general election, scheduled for 2029, as he attacked Reform UK's Nigel Farage, Tory leader Kemi Badenoch, and Green Party boss Zack Polanksi.