Minicab drivers are launching a legal action against the London mayor, Sadiq Khan, accusing him of discrimination against a largely ethnic minority workforce by making them pay the congestion charge while black cab drivers, who are overwhelmingly white, will be exempt.

Khan oversees Transport for London (TfL), which is planning to levy a £12.50 ultra-low emission zone (ULEZ) charge to drive in the city centre from 8 April as “a key part of our plans to both reduce congestion and to protect Londoners from harmful emissions”. But the Independent Workers Union of Great Britain (IWGB), which represents many Uber drivers and other gig economy workers, claims it amounts to indirect discrimination and violates the human rights of its members.

Until now, minicabs and black cabs have been exempt from the existing £11.50 congestion charge. When the ULEZ charge is levied, the cost to about 18,000 minicab drivers estimated by City Hall to carry passengers in central London each day is likely to be more than £1.4m a week.

It could push down earnings for Uber drivers, which have been estimated by academics at about £11 per hour, by about 13% over an eight-hour shift. Many drivers say they already earn less than the London living wage of £10.55 per hour.

Black cab drivers will not have to pay the charge because, City Hall said, their vehicles were accessible to disabled people and because they had a legal obligation to accept fares and to take the shortest route.

Only 8% of private hire drivers are white British compared with 80% among black cab drivers, according to those who answered a survey by TfL. The IWGB has issued a pre-action letter to Khan giving him until 6 March to reverse the decision before it applies to the high court for a judicial review. It argues the decision is in breach of the UK Equality Act and the European convention on human rights that protects property rights and which has been interpreted as protecting people’s ability to carry out their profession.

The secretary of the IWGB’s private hire drivers branch, Yaseen Aslam, urged Khan to remember his heritage as the son of a Pakistani immigrant, and said: “I didn’t imagine that I would be forced to take legal action against mayor of London, Sadiq Khan, for introducing a policy that discriminates against our community. We hope the mayor sees sense and scraps this policy that promises to push thousands of drivers into deeper poverty.”

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“It’s completely discriminatory that we have to pay this charge while black cabs continue to be exempt,” said Muhamed Ali, a minicab driver and branch committee member. “If Sadiq Khan doesn’t reverse this policy we will continue to fight it in the courts and in the streets.”

A spokeswoman for the mayor said: “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. Sadiq simply isn’t prepared to ignore the damaging impact this has on congestion and increasing air pollution. Congestion has a crippling impact on businesses across the capital.

“At the same time, our toxic air in London is a major public health crisis that is stunting the lung development of our children, leading to thousands of premature deaths, and increases the risk of asthma and dementia.”

City Hall said that private hire drivers who wished to avoid the charge could switch to a low-emissions vehicle or drive a vehicle that was accessible for wheelchairs.