A legal challenge to the expansion of London’s ultra-low emission zone will be heard in the high court later this year, after a judgment permitted councils to proceed. The city’s mayor, Sadiq Khan, vowed to press on regardless with plans to extend the Ulez, which he has argued is needed to tackle toxic air that is responsible for thousands of premature deaths a year. Four London borough councils – Hillingdon, Bexley, Bromley, Harrow – and Surrey county council were granted permission to challenge the policy in court, although only two of the five grounds they presented were deemed “arguable”, and only in part. The Ulez, which is aimed at reducing air pollution by charging drivers of older vehicles, will be expanded from inner boroughs to the whole of Greater London in August. The five Conservative-led councils have sought to block the expansion, which is unpopular among many drivers in the suburbs. Residents and small businesses have expressed concern about the financial impact, although Transport for London estimates that only a small minority of older vehicles will be liable for the charge, which is £12.50 daily for diesels typically registered before 2015 and petrol cars before 2004. The councils argued that Khan had failed to comply with relevant statutory requirements and had not consulted on a £110m scrappage scheme introduced to help drivers transition to newer cars or public transport. Sir Ross Cranston, a former high court judge, said the first ground was “doubtful but arguable” and that the second was “on the cusp”, although only on one aspect of its argument. He turned down three other claimed grounds for appeal. The court hearing is expected in July. A spokesperson for Khan said: “The mayor is pleased to see the court has refused permission for the majority of the grounds. We will continue to robustly defend his life-saving decision to expand the Ulez and continue with preparations without delay. “It is a shame that some local authorities have chosen to attempt this costly and misguided legal challenge instead of focusing on the health of those they represent. This is a health emergency and the mayor is not prepared to stand by and do nothing.” Nick Rogers, the transport spokesperson for London assembly Conservatives, said: “The high court has now ruled there is sufficient evidence that Sadiq Khan’s Ulez decision may have been unlawful. “Sadiq Khan should do the right thing, immediately stop work on his Ulez expansion, and explain his actions to the court.” Plans for the Ulez were originally drawn up under Boris Johnson, but expanded significantly under Khan from central London to the north and south circular roads in October 2021. TfL and City Hall say fewer than 6% of vehicles driven in the zone are now non-compliant, cutting air pollution by a quarter. Critics of the Ulez expansion argue that there are fewer public transport alternatives in outer London. Khan has promised to increase bus services, including a recently launched express service connecting outer boroughs, the Superloop.