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The Conservatives have narrowly held off Labour in Boris Johnson's former constituency, with the by-election loss being blamed on the expansion of a tax on polluting vehicles.
The Tories held off Labour in Uxbridge and South Ruislip by just 495 votes.
Both parties suggested plans by Labour Mayor Sadiq Khan's to expand London's Ultra Low Emission Zone (Ulez) played a part in the defeat.
He plans to make the clean-air zone three times bigger from 29 August.
Mr Khan has said widening the scheme will improve London's air quality, but it has proved hugely contentious in outer London boroughs, including Uxbridge and South Ruislip.
Labour's candidate in the constituency, Danny Beales, had said he wanted the expansion halted.
In his victory speech, new Tory MP Steve Tuckwell said Mr Khan had cost Labour the seat. Mr Tuckwell opposes the expansion, which he says will put an additional financial burden on residents.
"It was his damaging and costly Ulez policy that lost them this election," he said.
"This wasn't the campaign Labour expected and Keir Starmer and his mayor Sadiq Khan need to sit up and listen to the Uxbridge and South Ruislip residents."
Labour shadow justice secretary Steve Reed acknowledged it had been a factor in the campaign.
"I think there's been a number of issues at play, but there has certainly been a number of voters who have said to us that they are very concerned about Ulez," he told the PA news agency.
"Everyone wants to see clean air. But for some people, I think, given the chaos that there is in the economy, because the Conservatives have crashed it and the cost-of-living crisis that they fuelled, that this is the wrong time to introduce a charge for Ulez."
An unnamed Labour spokesperson told PA it was unsurprising that Ulez was a concern given that Conservatives "crashing the economy had hit working people hard".
They added Uxbridge was always going to be a difficult battle as the seat had never had a Labour MP.
The Ulez was introduced in central London in 2019, expanding a previous low emission zone for larger vehicles like buses, lorries and coaches that was first introduced in 2008.
Under Mr Khan, it was extended to cover the area within the North and South Circular roads in 2021.
Under the London mayor's proposed new expansion, its outer borders would reach Buckinghamshire, Essex, Hertfordshire, Kent and Surrey.
Drivers must pay £12.50 per day if they drive a vehicle which doesn't meet modern emission standards: generally, diesel vehicles that are more than seven years old, or petrol vehicles that are more than 17 years old. If they do not pay they face a maximum £180 fine.
Sir Keir Starmer has refused to take a side in the dispute over whether to expand Ulez.
In a BBC interview earlier this month, the Labour leader declined to say which view he backed.
He added that Mr Khan was trying to fulfil his legal obligations to reduce emissions, whilst Mr Beales was trying to fight for potential constituents.
"Both of those things have to be accommodated," he said.
The by-election took place in Uxbridge and South Ruislip after Boris Johnson decided to step down as an MP last month.
Despite disappointment in failing to take the outer London constituency, Labour made history by overturning its biggest-ever majority at a by-election in Selby and Ainsty.
The Liberal Democrats won the Somerton and Frome by-election by more than 11,000 votes.