(Bloomberg) -- The UK government’s plans to scrap London’s latest clean air policy hit a roadblock, just weeks before the mayoral elections.

Mayor Sadiq Khan’s widening of the city’s Ultra Low Emissions Zone last summer to the outermost parts of London has become a political flash-point among voters, with some protesters seeking to destroy cameras that monitor compliance. In July, it cost Khan’s Labour Party a chance at winning former prime minister Boris Johnson’s parliamentary seat in the outer London borough of Uxbridge.

Gareth Johnson, an MP for Dartford, which borders London, proposed a private members bill requiring Transport for London to seek government permission to amend the boundaries of any clean air zones. A spokesperson for Prime Minister Rishi Sunak confirmed the government supported the bill. 

However, efforts to pass the legislation before a general election, due by January 2025 at the latest, were stalled by Labour members on Friday, who ran down the time allocated to debate the bill. To proceed, the government will need to set aside additional parliamentary time, which Sunak’s spokesperson did not confirm would happen.

Read more: Why ULEZ Car Pollution Charge Has Londoners Divided

Still, the ULEZ — an area of London where drivers of high-polluting vehicles have to pay a charge to drive in — could become a dividing line in London’s May 2 mayoral election. The Conservative mayoral candidate Susan Hall has vowed to scrap the expansion if she wins. 

Khan has maintained the plan is helping to bring pollution down to safe levels while having a limited effect on drivers. 

A spokesperson for the mayor said the proposed changes by the Conservatives could undermine the powers of London’s local authority with little impact on motorists because 95% of vehicles on the city’s roads are now in compliance with the new rules and don’t pay the £12.50 ($16) daily charge.

“The mayor is clear that it’s up to other towns and cities to decide what works best locally for them,” Khan’s spokesperson said. “Ministers risk fundamentally undermining devolution if they start seizing powers from directly-elected mayors.”

The decision by the government to back the bill shows how much the Conservative Party has changed its position on the ULEZ, first introduced when Boris Johnson was mayor of London. The government also promoted clean air zones in 2020 and 2023 and said that the ULEZ was the purview of the capital’s mayor.

--With assistance from Isabella Ward.

(Updates with details on bill stalling in Parliament)

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