The High Court is set to publish a ruling on Friday morning over the London mayor’s plan to expand the Ultra Low Emission Zone (ULEZ) to the capital’s outer boroughs.

Five Conservative-run councils launched legal action back in February over the extension, which is due to come into force on 29 August.

The scheme – already in place in central and inner London – will see the drivers of the most polluting vehicles charged £12.50 a day to use them.

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The hope of those behind the plan is it will incentivise people to use cleaner transport alternatives and, as a result, help improve the city’s air quality.

And TfL has claimed only a small number of people will be impacted, with nine out of 10 vehicles compliant with ULEZ requirements.

But the councils challenged the rollout in the courts, saying the capital’s Labour mayor, Sadiq Khan, had exceeded his legal powers with such a large expansion of the scheme.

The four local authorities – Hillingdon, Bexley, Bromley and Harrow in London, plus Surrey County Council – also claimed the consultation on the plan was flawed, and not enough information had been shared over the scrappage scheme, which provides payouts to people prepared to ditch their vehicles.

While other parts of the challenge were dismissed in April, the councils were granted a hearing in the High Court, and the two sides fought it out over two days of evidence.

The court’s final decision is expected at around 10am on Friday.

A spokesperson for Mr Khan said: “The mayor took the very difficult decision to expand the ULEZ London-wide so that five million more people in our city can breathe cleaner air.

“We await the decision from the judge on the future of this important scheme.”

The ruling comes a week after the debate around ULEZ dominated a by-election and the fallout from the results.

The seat of Uxbridge and South Ruislip – left vacant by the departure of Boris Johnson – seemed ripe for the taking for Labour in light of recent polling that gives the party a double-digit lead over the Tories.

But the Conservative candidate managed a narrow victory – albeit seeing the majority for the party fall from over 7,000 to less than 500 – having campaigned heavily against the expanding Ultra Low Emission Zone.

Since then, Labour have been in turmoil over the policy and whether to support it, with Sir Keir Starmer saying he had asked the mayor to “reflect” on the impact of the scheme.

However, Mr Khan has said he is committed to ULEZ expansion, telling Sky News: “It was a difficult decision to take. But just like nobody will accept drinking dirty water, why accept dirty air?”