Updated

New York City's Taxi of Tomorrow is under attack.

First a group of fleet owners filed a lawsuit against the city, arguing they shouldn't be forced to buy just one type of vehicle, now the city's comptroller says NYC should reject the exclusive contract with Nissan because its NV200 is not wheelchair accessible.

"In our view, this contract ignores the civil rights of New Yorkers who use wheelchairs by failing to make wheelchair-accessible vehicles," said Liu, a likely candidate in next year's race to succeed Mayor Michael Bloomberg.

It was unclear whether Liu's action would have any effect, as Bloomberg said on his WOR radio show Friday that the program would go forward over Liu's objections. "We can go ahead and do it anyway, which we will," he said.

City officials approved a plan in September to make the Nissan NV200 minivan the new yellow cabs. The Nissans will replace the more than 13,000 existing cabs as they age and are retired. They will be phased-in beginning in the fall of 2013.

The Taxi and Limousine Commission has said that people in wheelchairs can call 311 to get an accessible taxi.

Wheelchair users at Liu's news conference said that's not good enough.

"We don't want a system that's special, we don't want a system that's separate," said Edith Prentiss, chairwoman of the Taxis for All Campaign. "We want a system in which we can go out and put our hand in the air like every other person."

Read: Smartphones can soon be used to 'e-hail' NYC taxis

Liu said yellow cabs "are a worldwide symbol" of New York City, but just 231 of them are accessible to those in wheelchairs.

"For wheelchair users, our taxi fleet is a separate but unequal system, and it is offensive to the inclusive spirit of New York City," he said.

The Associated Press contributed to this report