Keir Starmer has refused to say whether he backs Sadiq Khan's expansion of the London Ulez (Ultra-Low Emission Zone) – after a Labour candidate said it should be halted.
Danny Beales, who is standing for the opposition in this month's Uxbridge by-election, said this week that it was "not the right time" to expand the driving charge for the most polluting vehicles.
The £12.50 levy has come up on the doorstep in the outer-London battleground seat, which will vote on 20 July on who should replace Boris Johnson as its MP.
In an interview with the BBC Sir Keir said Mr Khan was bound by the "context" of legal obligations to cut pollution, whilst he said Mr Beales was "fighting for what he hopes will be his constituents".
"Both of those things have to be accommodated," Sir Keir told the broadcaster.
Sir Keir had said earlier this year that Mr Khan was "right" to bring in the policy.
But repeatedly asked on Thursday by the BBC whether he agreed with his by-election candidate or Mr Khan's position, the Labour leader declined to say.
"I want the candidate to fight for his, hopefully, constituents. And the mayor obviously has to work within the legal constraints that are imposed on him," Sir Keir said.
When again pushed on what his own view was, Sir Keir said: "I think both of those things have to be accommodated."
Under the Ulez cars that do not meet pollution standards have to pay an additional £12.50 daily charge to drive within the north and south circular roads.
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Show all 50But under Mr Khan's expansion plans, from 29 August the charge will be expanded to the whole of Greater London.
Polling conducted by YouGov last year found that 51 per cent of Londoners supported the expansion of the clean air zone compared to 27 per cent who do not.
But the policy is thought to be less popular in outer London – where the Uxbridge seat is located.
The episode comes after the Labour leader was accosted by angry students on Thursday urging him to commit to more climate action and stop making "U-turns".
The protesters, college students in Kent, wanted Sir Keir to reinstate a scaled-back plan to invest £28 billion a year in a Green New Deal.
