“It’s a problem for the city,” said Mr. Daus, now a partner at the law firm Windels Marx Lane and Mittendorf, where his focus is transportation law. “This is a real, credible threat.”

He said that the administrative code had been written narrowly, explicitly calling for a “hybrid electric vehicle” option, not simply a fuel-efficient car. The city has argued that the yellow taxi fleet’s gas mileage will increase once the Taxi of Tomorrow is widely used.

An administration official, who requested anonymity because the city’s legal strategy had not been determined, suggested a possible counterargument: the code requires the commission to approve a hybrid vehicle “within 90 days after the enactment of this law,” which the administration did. Taken literally, the official said, the provision could be interpreted to apply only to the moment of the bill’s passage in 2005, even if hybrid alternatives would not be available once the Taxi of Tomorrow was fully implemented.

If such logic fails, it is unclear how the city might expect to circumvent the provision while keeping its exclusive commitment to Nissan. (The company would not comment on the lawsuit or its contract with the city.)

The City Council has the authority to amend the code, but seems unlikely to provide a solution. Last September, Councilman James Vacca and Christine C. Quinn, the Council speaker and a presumptive Democratic front-runner for mayor, wrote a letter to Mr. Yassky expressing concerns about “reducing the number of green taxis on our streets.”

The city has attracted myriad lawsuits over its taxi policy in recent years, facing attacks from well-financed corners of the yellow and the for-hire taxi industries. Last June, a State Supreme Court justice in Manhattan blocked the city’s all-borough taxi plan, passed by the State Legislature in 2011 after the administration had been rebuffed by the City Council. The judge rejected the city’s justification for the maneuver — that taxi policy is of state concern.

With taxi-hailing apps set to begin for yellow taxis as soon as this month, for-hire owners have retained the counsel of Randy M. Mastro, a former deputy mayor for operations under Rudolph W. Giuliani, in preparation for a possible suit. Livery and black-car operators, concerned about their business model, have protested that the apps violate the city’s longstanding ban on prearranged rides in yellow taxis.