Sadiq Khan has reportedly called on Andy Burnham to lay the groundwork for Britain to rejoin the European Union - arguing that the case should be made well before the next general election.
In a press interview ahead of the Labour Party Conference in Liverpool, the Mayor of London said he believed the government should move beyond its current Brexit settlement and prepare the public for closer ties with Brussels.
Such a move could include a possible return to the customs union and ultimately EU membership, I News reports.
Sir Sadiq told the newspaper: "We’ve got to start the work now… You can’t, a week before a general election, pull out of a hat this rabbit of a manifesto and say we’re going to rejoin in six weeks’ time.
"The heavy lifting, the pitch rolling must be done now, explaining to the country the disbenefits of being outside the European Union and the benefits of being part of this family."
Khan has previously referred to Brexit as the “biggest act of economic self-harm in the UK’s history”.
But his latest intervention places the London mayor among the most outspoken senior Labour figures on the issue and goes further than recent comments from former prime minister Tony Blair, who argued that Britain should work towards rejoining the bloc over the coming decade.
The comments also come amid renewed debate within Labour over the UK's future relationship with Europe.
Prime Minister Andy Burnham has signalled greater openness to closer engagement with the EU, although ministers remain divided over how far and how quickly relations should develop.
Mr Khan, a longstanding critic of Brexit, described leaving the European Union as a major economic mistake and said politicians should begin explaining the benefits of membership to voters now, rather than waiting until the next election campaign.
He argued that any future attempt to rejoin would require years of preparation and public discussion. The mayor said the economic and trading links between Britain and Europe made a strong case for closer alignment, pointing to the EU's status as the UK's largest trading partner.
Under the approach outlined by Mr Khan, the Government would first continue efforts to improve relations with Brussels before seeking closer economic integration.
He suggested that rejoining the customs union should be considered in the near term and said options including single market participation and a form of associate membership should also be explored.
The mayor argued that Labour's next election manifesto should set out a clearer long-term ambition for Britain's relationship with the EU. He also suggested that a future general election victory could provide a democratic mandate for rejoining the bloc.
His intervention is likely to prove controversial, with opponents arguing that reopening the Brexit debate risks reigniting political divisions that have dominated British politics for much of the past decade.
Several ministers are understood to be cautious about committing to rejoining the EU, with some questioning whether such a move would be politically or practically achievable.
Mr Khan rejected suggestions that Labour should be bound by positions adopted before recent geopolitical and economic developments, arguing that governments must respond to changing circumstances.
The Mayor of London has consistently campaigned for closer relations with Europe since the 2016 referendum, in which London voted strongly in favour of remaining in the European Union. His latest comments are likely to add further pressure on Labour as it seeks to define its long-term approach to Britain’s place in Europe.
