Hundreds of taxi drivers claim they could be put out of business by a bid to reduce pollution in Birmingham.
The government has said Birmingham must be among six English cities with a Clean Air Zone by 2020.
The council proposes licensed vehicles must meet EU standards on emissions by December to have licenses renewed.
The authority estimates 1,428 private hire cars and 530 hackney carriages would fail the test. Drivers say their livelihoods are being threatened.
The city's fleet is 4,200 private hire cars and 1,233 hackney carriages.
The penalty for not meeting standards is not being allowed in the city or being charged to enter.
Drivers say they can either pay £8,000 to convert cars to use liquid petroleum gas (LPG) or buy electric taxis costing around £40,000.
Raja Amin, from the RMT union, said the plans "jeopardised drivers' livelihoods" and drivers were being made "scapegoats" as there were no proposals to police emissions from other types of vehicle.
If it does not achieve the Clean Air Zone in time, the council estimates it could face a £60m fine.
All drivers wanting to renew licences by December 2017 will have to meet the Euro 4 standard for petrol driven cars or Euro 5 for diesel.
From December 2018, the council proposes to strengthen its policy so hackney carriages meet at least a so-called Euro 6 standard or private hire drivers use Ultra-Low Emission Vehicles.
The Department for Transport has allocated £500,000 to cover conversion costs for 63 hackney carriages.
Drivers can also apply for a grant of up to £7,500 towards a new taxi.
The council said: "The city has a very old fleet of hackney carriages and private hire vehicles so we need to look at how we can solve this issue to the benefit of all.
"We will do everything we can to shape the market for cleaner vehicles and assist drivers."
The other cities required to have a clean air zone are London, Leeds, Nottingham, Derby and Southampton.
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