Addison Lee has said Sadiq Khan's plans to remove the congestion charge exemption for private hire taxi companies could cost it £4m a year.
Uber rival Addison Lee has commissioned research that suggests levying a congestion charge on minicabs in London would make the capital more congested and polluted.
Oxera Consulting, which carried out the research, also said Transport for London' s (TfL) proposals could lead to a bill of £250 a month for drivers and drive down competition.
Taxis and PHVs currently are exempt from paying the congestion charge when actively licensed with London Taxi and Private Hire, but the exemption for PHVs only applies to private hire bookings.
The findings suggest that the charge could push customers back into using black taxis to avoid the charge and could force firms to scale back on green investment in order to pay the charge.
Last month the London Assembly overwhelmingly voted against Khan's plans to remove the congestion charge exemption for PHVs.
Addison Lee says a better way to reduce congestion and pollution would be through increasing the existing £11.50 charge to £12.50 a day.
It is understood that Addison Lee is considering taking legal action if Khan's consultation becomes policy.
Chief executive Andy Boland said:
Addison Lee is fully on board with TfL’s objectives of tackling congestion and improving air quality in London, and are investing £60m to meet the existing ultra-low emission zone (Ulez) policy. However, independent analysis suggests that far from helping to meet these objectives, imposing the congestion charge on private hire vehicles would make matters worse.
We believe our proposed policy of focusing on Ulez and closing its loopholes, a small increase in the overall charge and a proper rapid charging network would be fairer, simpler and more effective.
During the summer TfL said it would introduce a new 20mph speed limit on all of its roads within the congestion charging zone in a bid to reduce the number of people killed or seriously injured on the capital's roads by 65 per cent by 2020, with no-one being killed on or by a bus by 2030.
A spokesperson for Khan said: “The rules around private hire vehicles and the congestion charge were first put in place back in 2003 and are no longer fit for purpose.
“The number of private hire vehicles entering the congestion charge zone has shot up from 4,000 a day in 2003, to more than 18,000 now. The mayor is not prepared to standby and ignore the damaging impact this has on increasing air pollution, harming Londoners’ health and increasing congestion that can be so detrimental to London’s businesses.
“The mayor has been clear that the cleanest and most accessible vehicles will still be exempt from paying the congestion charge, meaning older and disabled passengers will still get the service the need, and they’ll be a further reduction in dirty emissions causing so much damage to Londoners’ health."
Khan also generated a backlash when he wrote to transport secretary Chris Grayling in the summer to demand new powers to cap the number of minicabs in the capital following a similar clampdown in New York.
The mayor said there were now more than 110,000 actively licensed drivers in London, an 83 per cent increase on seven years ago – numbers that some operators have disputed.
Richard Dilks, transport policy director at London First, said the calls for a cap were “crude”, while Steve Wright, chairman of the Licensed Private Hire Car Association, branded the plan “absolutely potty”.