Rishi Sunak is backing a bill that would overturn the expansion of London’s Ultra Low Emission Zone, in a move which sources close to the city’s mayor described as an “unprecedented assault on democracy and devolution.”

Powers over transport and air quality are currently devolved to the Mayor.

Londoners will also soon be handed the opportunity to have their own voice heard on the issue when they vote in May's London mayoral elections.

Sadiq Khan's Conservative candidate Susan Hall has made scrapping the zone's extension her central pledge, but is currently 24 points behind him, according to a Savanta poll published on Friday.

However, under the new backbench bill, which is being brought to Parliament today by the Kent-based Conservative MP Gareth Johnson, the Government would be handed the ability to unilaterally scrap the extension of the zone anyway.

The Transport Secretary Mark Harper said in a statement that the Government was "happy to support" Johnson's Bill.

“The government has been clear the Mayor of London’s decision to expand ULEZ charging area to the London borders, in breach of his own manifesto commitment, is a tax on the poorest motorists, which his own impact assessment states, in terms of air pollution, will only have a moderate impact on NOx and minor impact on particulates", Harper said.

A spokesperson for the Prime Minister added that the bill would allow "communities to have their say".

A source close to Khan hit out at the bid to overturn the zone's extension.

“This unprecedented assault on democracy and devolution is a desperate distraction by a Government in its death throes which time and again has shown its contempt for Londoners and their rights,” the source said.

“Londoners will see through this pathetic attempt to play politics with the capital.”

Downing Street had previously ruled out seeking to overturn Sadiq Khan’s decision to extend the city’s air quality zone to Outer London.

A spokesman for the Prime Minister said last month that "road user charging is a matter for the Mayor of London and for him to justify his decision to residents and businesses."

The zone, in which owners of higher-emission vehicles are compelled to pay a daily charge if they drive inside London’s boundaries, has proven controversial with some Londoners.

Labour leader Keir Starmer has also previously criticised it, telling Khan last summer that he should “reflect” on the policy. A spokesman for Starmer told Byline Times that the Labour leader's view had not changed since the scheme was brought in.

However, City Hall say the scheme has been a success, with a spokesman saying that 95% of vehicles on London’s roads were now compliant with the newly expanded zone, which was “helping clean up London’s air and protect Londoners’ health.”

While the Government's apparent support for Johnson's bill will allow it time in Parliament, it is unclear whether it will be given sufficient time to pass into law before the next general election.

Sunak's spokesperson said plans for the bill's passage would be set out by the Leader of the House in the coming weeks.

It comes as Conservative MPs also call on the Prime Minister to remove the Mayor’s powers over policing.

Asked this week about the push to reduce the Mayor’s powers, Khan told this paper that “you can tell there's a general election and a mayoral election around the corner because of these silly gimmicks and games from the Tories.

“They should get their own house in order before they start lecturing us about taking powers away.”

A spokesman for the Prime Minister did not respond to a request for comment by the time of publication.