A judicial review into Sadiq Khan's plans to expand London's Ultra-Low Emission Zone (ULEZ) is due to get under way at the High Court.
Five Conservative-led councils - Bexley, Bromley, Harrow, Hillingdon and Surrey - have launched legal action over the scheme, planned to cover the whole of London from the end of August.
They say it would have a big financial impact on motorists.
London's Labour mayor says the scheme is needed to tackle air pollution.
"The independent assessment confirms that ULEZ works and the expansion will lead to five million more Londoners breathing cleaner air," he told Reuters news agency.
"You're not going to please 100% of people all the time," he added. "No politician in history has managed to do so."
The ULEZ scheme requires people who drive in non-compliant, or more polluting, vehicles to pay a daily charge of £12.50 on days they are driven within inner London. Motorists could face a £160 fine if they do not pay.
A majority of cars driven in London are ULEZ-compliant.
The current zone covers all parts of the city between the North and South circular roads, but London's mayor previously announced it would be expanded to cover the whole of the capital from 29 August.
Lorries, buses, coaches and heavy vans which are non-compliant are charged £100 under a separate low emission zone scheme, which already covers most of London.
Speaking to BBC Radio 4's Today programme, Paul Osborn, the Conservative leader of Harrow Council, said the authority believed its expansion would have a "devastating impact on the poorest motorists in Harrow".
"People who do low paid jobs in antisocial hours, they don't have public transport alternatives. They're being asked to pay £12.50 every day to go to work and if they work over midnight, they'll be asked to pay £25 because they have to pay it for the next day as well," he said.
He added the money spent on the thousands of cameras that are being set up across London could be better spent on widening the scrappage scheme.
Transport for London says all ULEZ revenue is reinvested into running and improving the capital's transport network, such as expanding bus routes in outer London.