Labour will not be able to hit its target to invest £28 billion a year in green projects until the end of its first term, Sadiq Khan said after the party insisted it is not abandoning the key pledge.
The future of the pitch was cast into doubt by reports suggesting Labour is considering axing the expenditure commitment amid political pressure.
But shadow foreign secretary David Lammy insisted the party is committed to its green prosperity plan by the end of a first parliament, although “not whatever the economic climate”.
Sir Keir Starmer has previously hinted he could scale down the investment given the financial picture he would inherit if he becomes prime minister.
And shadow chancellor Rachel Reeves last year diluted the original promise to spend £28 billion annually from the first year of a Labour government, saying it would would instead be a target to work towards in the second half of a first parliament.
Climate-conscious London Mayor Mr Khan on Saturday admitted that level of spending will not be possible for some time.
Speaking at the Fabian Society conference in London, he said: “I understand in the first couple of years of a Labour government, Rachel will not be able to write a cheque for those sorts of sums.
“So we’ve got to give Rachel and the team time to clean the mess left by this Government.
“And then towards the end of the first term, hopefully there’ll be enough money to invest in this.”
He added: “Whether we get to £28 billion, it depends how quickly we can grow the economy.”
Mr Khan also said the UK should follow the example of US President Joe Biden’s Inflation Reduction Act, which has used government subsidies to boost green businesses.
Reports had suggested senior party officials will hold crunch talks on the plan’s fate and could abandon the £28 billion figure if they conclude it will damage the party in an election campaign.
Labour also failed to mention the commitment in a campaigning brief published earlier this week.
But Mr Lammy earlier denied meetings are taking place and repeated Labour’s commitment to the pledge.
He told BBC Radio 4’s Today programme: “That commitment to £28 billion to the green prosperity plan by the end of the next parliament is something that I think that you can trust the Labour Party to remain committed to.”
But, he added the caveats “not whatever the economic climate” and “it’s got to be within our fiscal rules”.