Transport for London (TfL) will require all foreign private-hire drivers—the sector in which Uber operates—to be able to prove proficiency in the English language if they do not come from a majority-English-speaking country.

The issue is what that proof must entail, and Uber thinks TfL is going too far by requiring written English tests.

The San Francisco-based company is also challenging the regulator’s requirements that Uber locate its customer service facility in London, give TfL a heads-up when it’s going to change its app or business model, and ensure that drivers are insured even when they’re not working.

Uber has sent a letter to TfL to inform the regulator that it will this week file a legal challenge to the new rules.

“This legal action is very much a last resort. We’re particularly disappointed that, after a lengthy consultation process with Transport for London, the goalposts have moved at the last minute and new rules are now being introduced that will be bad for both drivers and tech companies like Uber,” said Uber London general manager Tom Elvidge.

As for what Uber considers to be “the last minute,” TfL said back in March that it would require a formal English language requirement for all drivers, and then specified in June that this meant a valid certificate of English proficiency at the “B1” level, as classified under the Common European Framework system.

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This certificate can only be issued after the applicant takes a written exam. Uber said this exam costs £200 ($260) and is harder than the test for British citizenship, further arguing this requirement contravenes the U.K. Equality Act. Uber also complained this will slow down the roll-out of new features if it has to inform TfL every time it plans to change its app or model.

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TfL, meanwhile, said it needs to be able to check that changes are legal. The authority also said it is important that drivers have enough English language proficiency to be able to explain to their fares what needs to happen in an emergency as well as to be able to read information sent to them from the regulator.

“We responded to Uber’s letter and will be robustly defending the legal proceedings brought by them in relation to the changes to private hire regulations. These have been introduced to enhance public safety when using private hire services and we are determined to create a vibrant taxi and private hire market with space for all providers to flourish,” a TfL spokesman said in a statement.

Addison Lee, an established private-hire company that is also no friend to Uber, has written to London mayor Sadiq Khan to protest against Uber’s legal action and back up TfL’s new regulations.