If Mayor Sadiq Khan's plan goes ahead, London's Ultra Low Emission Zone (ULEZ) will become one of the world's largest to tackle air pollution, encompassing 5 million extra people in the capital's leafier and less-connected outer boroughs.

His plan - which echoes hundreds of others in place in traffic-choked cities across Europe - will be tested in the courts on Tuesday when local councils challenge the expansion, arguing due process was not followed, especially in how consultations were carried out.

The expansion has triggered a fierce debate across the city, pitching the mayor and health campaigners, who point to the environmental success of the initial ULEZ restrictions, against those who say they can't tolerate another economic hit at a time of soaring living costs.

Carl Cristina, a 44-year-old tree surgeon living just outside the boundary, says it will wreck his livelihood.

"I can't afford to buy a new van. I simply don't have the budget," he said, speaking to Reuters at a central London anti-ULEZ demonstration attended by hundreds.

He dismisses a scrappage scheme which could lower the cost of a replacement vehicle and says his only choice would be to pass the charge on to customers - something he believes would make him uncompetitive and cut his income in half.

London's transport authority says only one in 10 cars in outer London are not ULEZ-compliant. But like much of the data around the expansion, that has been heavily contested with some saying the true figure could be higher.

Cristina has applied for a job in the rail sector.

CAN'T PLEASE EVERYONE

Simplified as an economy-versus-environment debate, the London row mirrors challenges policymakers around the world are facing, as the rapidly closing window to halt a catastrophic rise in global temperatures rubs up against the cost of action.

In Rome this month the mayor delayed plans to toughen curbs on polluting vehicles after protests from citizens groups.

Khan, who is running for a third four-year term as London's mayor, acknowledges the need for support - pointing to a 110 million pound scrappage scheme to subsidise the cost of a newer vehicle by 2,000 pounds, and a list of exemptions, including for disabled people.

"The independent assessment confirms that ULEZ works and the expansion will lead to 5 million more Londoners breathing cleaner air," Khan told Reuters in an interview.

He pointed to research showing the introduction of the ULEZ in 2019 caused nitrogen dioxide levels to fall by nearly half in central London, and that the assessment had overall backed an expansion. Critics dispute the assessment's conclusion.

Less than a year away from a mayoral election, irate callers are lighting up the switchboards on the capital's radio phone-ins. Public cameras installed to enforce ULEZ have been vandalised. Other opponents have threatened disruptive protests.

But Khan, who was diagnosed with asthma and wrote a book this year on air pollution and climate change, says he is determined to face down his critics.

"You're not going to please a hundred percent of people all the time," he said. "No politician in history has managed to do so."

Some experts say calculating the net benefits of a such an expansion can be complicated.

Low emission zones make a lot of sense in city centres, where air pollution is higher and many public transport alternatives exist, said Thomas Verbeek, assistant professor in urban studies at Delft University of Technology in the Netherlands.

"But the further away from the city centre you go, the less you can improve air quality," Verbeek added.

CAN-KICKING

A YouGov poll last year showed 43% of Londoners supported the planned expansion, while another 8% supported a delayed one. About 27% were opposed and the rest undecided.

Jemima Hartshorn, founder of campaign group Mums for Lungs and mother to an asthmatic daughter, has no doubts, saying it is often the poorest who suffer by living next to busy roads.

"It's absolutely critical that even in a cost-of-living crisis we do not kick the can of air pollution down the road and let more children grow up unhealthy and unwell," she said.

But others are focused on the economic harm it could cause if it stops shoppers, diners and workers like tree surgeon Cristina from coming into the city.

Teresa O'Neill, leader of one outer London council behind the court challenge, said local businesses such as care agencies were fearful of losing staff while those in food and retail were worried of a fall in demand, as ULEZ drives up costs.

"I've been leader now for 15 years ... and I don't think we've ever had an issue like this that has actually garnered so much attention," she said. "People tell you they absolutely hate it."