London mayor Sadiq Khan has told Tory councils opposed to his extended clean air zone they need to back down.
The Labour mayor is locked in an increasingly bitter battle with some local authorities over his plan to include all of Greater London in the Ultra Low Emission Zone.
Four Conservative-controlled outer London boroughs, who want the August launch postponed, are considering a legal challenge.
They have been joined by Buckinghamshire, Essex, Kent and Surrey county councils which are refusing to allow ULEZ signs warning motorists as they approach the London boundary.
But the mayor said council leaders would be to blame if drivers of older cars and vans were forced to pay a fine of up to £180.
"What will happen is their constituents, their residents, won’t realise they’re entering London and will inadvertently pay a fine," said Mr Khan.
He continued: "Their residents will know the only reason they weren’t aware they were entering the ULEZ area is because their councils didn’t bother to put up signs."
The mayor claims research by Imperial College London in 2019 showed 4,000 premature deaths in the capital due to toxic air.
An estimated 200,000 cars and vans driving in the 32 London boroughs fail to meet modern emissions standards.
Their drivers would have to pay a daily £12.50 Ulez charge or face a fine.
Councils opposed to the expansion claim the cost of living crisis means many motorists cannot afford to buy newer, cleaner vehicles.
They’re also demanding the mayor make a £110m scrappage scheme announced last week more generous.
Londoners on some benefits, charities and small businesses can apply for grants but car and van owners in the Home Counties are excluded.
Mr Khan added: "I’m quite clear, the Conservative councils need to back down in their opposition to clean air."
Cllr David Brazier, Cabinet member for Highways & Transport, Kent county council said: "If the mayor won’t come to the table, then we could simply say we are not going to allow you to place signs in Kent’s territory and if you wish to warn people that they’re approaching the Ulez then you will have to provide those signs within your own territory."