Sir Keir Starmer has called on Labour’s mayor in London to “reflect” on his decision to expand the Ultra Low Emission Zone (ULEZ) to the capital’s outer boroughs, after the impact of the policy on the Uxbridge and South Ruislip by-election.

Many had predicted Labour would overturn the 7,000 majority in Boris Johnson’s old seat in the west of the city after the former prime minister’s shock exit as an MP last month.

But throughout the campaign, it became clear Sadiq Khan’s plan for ULEZ had angered people on the doorstep.

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The Tories clung on to the seat with a majority just shy of 500 votes – which was still a 6.7% swing towards Labour – and Rishi Sunak told reporters: “When there’s an actual choice on a matter of substance at stake, people vote Conservative.”

Speaking to reporters, Labour leader Sir Keir said the constituency was always “going to be tough” to win as it had never voted for his party.

But he said the ULEZ expansion was also a factor in “why we lost in Uxbridge”, adding: “We all need to reflect on that, including the mayor.”

Mr Khan also said he was “disappointed” the party didn’t win the seat overnight.

But he insisted the decision to widen the ULEZ, due to come in next month, was “the right one”, adding: “It was a difficult decision to take. But just like nobody will accept drinking dirty water, why accept dirty air?”

The ULEZ was first proposed by Mr Johnson during his stint as London mayor as a way to cut air pollution in the capital.

When he announced it in 2015, he said it was “an essential measure to help improve air quality in our city, protect the health of Londoners, and lengthen our lead as the greatest city on earth”.

The policy in its current form – which sees drivers having to pay a £12.50 daily fee to drive in the zone if their car does not meet emissions standards – currently covers central London and the areas up to, but not including, the North and South Circular Roads.

But Mr Khan plans to expand the zone up to the capital’s borders with Buckinghamshire, Essex, Hertfordshire, Kent and Surrey from the end of August.

The mayor defended the by-election loss, saying: “I have lived in London my entire life [and] this seat has never been Labour since I’ve been alive, including in 1997 in the Tony Blair landslide and the subsequent by-election a few months later.

“But I am quite clear though, the policy to expand the Ultra Low Emission Zone is the right one.”

Mr Khan outlined the successes of the existing policy, saying there had been a toxicity reduction in central London of 50% since ULEZ began, and 20% for inner London boroughs.

He also said a third fewer children were being admitted to hospitals with asthma attacks.

“Four million Londoners are already benefiting,” the mayor added. “What about the other five million in outer London, where there is the largest number of premature deaths?

“The 10 boroughs with the largest number of premature deaths are in outer London [and] it is the poorest Londoners who are least likely to own a car that suffer the consequences.”

Mr Khan said he had been “listening” to people’s concerns about the expansion, increasing the eligibility of people to be exempt – such as families on child benefit or small businesses who employ up to 50 people.

“We are going to carry on listening, making sure we monitor the situation,” he added.

“But the choice is simple. Do we kick the can down the road to clean up the air in London or do we take action?”