Popular taxi and sedan service Uber is charging an automatic 20 percent tip on cab rides booked through its app or website.

The DC Taxicab Commission says that's illegal -- but so far the agency can't do anything about it.

The District's top taxi regulator, Ron Linton, told cabbies last month that Uber shouldn't be doing that and that he had asked them to stop.

"We are surprised as we had been assured by their attorney that Uber would comply with the regulations to prohibit charging a mandatory gratuity," Linton said in an email Friday.

Uber says it doesn't think the automatic tip is illegal.

"We don't believe that it is illegal to include a gratuity -- which goes entirely to the driver," said Uber General Manager Rachel Holt. "Uber customers know about the policy before they use our product since information about the tipping policy is on our blog, website and in the email we sent to all customers when we rolled out the product in DC."

Two other apps for taxi booking, myTaxi and Taxi Magic, don't charge automatic tips.

"We feel it's really important to put that actual tipping power in the hands of the user," said Taxi Magic spokesman Matt Carrington.

But another taxi app startup, HitchRides, does.

"We have found that consumers like the convenience of getting to their destination and simply getting out of the car and getting on with their business," said the company's founder, David Miller. "When you have to figure out a tip, it's an extra step in the process."

App developers said whether they're allowed to charge a tip is still a gray area in D.C. law, despite the Taxicab Commission's pronouncements.

The commission said it won't be able to do anything about the tips until a rider complains about them, at which point the commission will seek a refund for the rider and fine the driver, Linton said.

Uber ran into trouble with D.C. law last year, when Linton said the company's sedan service was illegal and performed a sting operation to slap a driver with a $1,000 fine. The D.C. Council since passed legislation allowing the town car service -- which charges customers using a blend of taxi and limousine methods -- and exempting it and similar sedan services from fare regulation.