The central London congestion charge first launched in February 2003 as a means to reduce traffic, encourage the use of public transportation, and reduce greenhouse emissions.

What Independent Worker’s Union of Great Britain (IWGB), which issued the letter and has also launched a fundraising campaign, is not happy about is that black cabs are exempt from paying the fee that private drivers have been asked to pony up for come April.

Today we launch a huge case vs Sadiq Khan to #DriveOutDiscrimination

Making minicabs pay congestion charge needlessly attacks the poorer side of the taxi trade while doing little to address congestion

For justice, we need your help. Pledge'n'share now!https://t.co/rXtlXbA5nM

— IWGB (@IWGBunion) March 1, 2019

A government report from Transport for London that was published in December found that 94% of minicab, Uber, and other hired drivers are people of color. According to the report, “many are from deprived areas, there is a disproportionate impact on these groups.”

IWGB wrote in its letter that a whopping 71% of black cab drivers, who would be exempt from the tax, “come from a ‘white’ background.”

Yahoo notes that a majority of black cab drivers are both white and of British descent—compared to just 6% of private drivers.

Sad for us to have to sue @SadiqKhan – a Labour Mayor, human rights lawyer & son of a migrant worker – for discrimination. But we couldn't allow institutional racism at TfL to win. If we didn't stand up to @MayorofLondon for minicab drivers who would? https://t.co/TnccRXMkoC

— UPHD (@United_PHD) March 1, 2019

IWGB secretary Dr Jason Moyer-Le told the BBC that the plan was “regressive” and “both discriminatory and fundamentally unfair”—potentially violating the Equality Act 2010 that protects against indirect discrimination.

When asked about the backlash, a spokesperson for the mayor told CNBC that Khan “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.”

Khan didn’t address the discrimination charges, but did tweet about how pollution was impacting London Friday.

London has faced harmful levels of pollution for much of this week. We’re taking a range of bold actions to tackle our city's toxic air, and cleaning up our bus fleet is just one. pic.twitter.com/42j6jhZqlM

— Mayor of London (@MayorofLondon) March 1, 2019

Mayor Khan has had a complicated relationship with Uber, calling on his right to cap the number of drivers in London.

IWGB’s letter gave Khan until March 6 to resolve the policy before the union takes the issue to the High Court for examination.