Updated, 3:52 p.m. | Dealing a sharp blow to the Bloomberg administration’s attempt to reduce air pollution and greenhouse gas emissions, a federal judge on Friday blocked New York City from requiring owners and operators of yellow taxicabs to switch to more fuel-efficient hybrid vehicles that operate on a mixture of gasoline and electricity.

The 26-page ruling [pdf], by Judge Paul A. Crotty, bars the city from implementing the hybrid-cab requirement, which was to take effect on Saturday and would have resulted in a virtually all-hybrid fleet by 2012. Taxi owners had maintained that fuel economy and vehicle emissions standards were the principal purview of the federal government.

The hybrid-cab requirement was a centerpiece of Mayor Michael R. Bloomberg‘s PlaNYC 2030 package of environmental initiatives. In a statement on Friday, he said, “We are very disappointed in the decision and we are exploring our appellate options.”

The mayor made it clear that he strongly disagreed with the ruling, which he said could cripple efforts by other cities to combat air pollution. “The decision is not a ruling against hybrid cabs,” he said, “rather a ruling that archaic Washington regulations are applicable and therefore New York City, and all other cities, are prevented from choosing to create cleaner air and a healthier place to live.”

The city has 13,237 yellow taxicabs. Nearly 1,500 of them are already hybrids as a result of voluntary efforts; a fraction of those hybrids are attached to medallions that were specifically auctioned for use with alternative-fuel vehicles.

The rule that was to take effect on Saturday was adopted by the Taxi and Limousine Commission on Dec. 11, 2007. It would have required that all new taxicabs coming into service achieve a fuel-efficiency city rating of 25 miles per gallon or higher, rising to 30 m.p.g. by Oct. 1, 2009. (The gasoline-powered Ford Crown Victoria, which in the late 1990s supplanted the Chevrolet Caprice as the workhorse of the city’s taxi fleet, gets about 12 to 14 m.p.g.)

Because all taxicabs must be replaced every three to five years under city rules, the new regulations would have resulted in a virtually all-hybrid fleet by 2012.

In their lawsuit, filed in September, taxi operators — led by the Metropolitan Taxicab Board of Trade, which represents the owners of 29 large fleets that control 3,500 yellow cabs, about a quarter of the fleet — said that the hybrid vehicles, which are more fuel-efficient, were not designed to withstand the heavy wear and tear that cabs must endure.

In his ruling, Judge Crotty, who was the city’s corporation counsel from 1994 to 1997, under Mayor Rudolph W. Giuliani, said he agreed to block the city from enforcing the rule because the plaintiffs were likely to succeed in their key legal argument — that the new regulations were pre-empted under federal law, which reserve regulation of fuel economy and emissions standards to federal agencies.

Though Mr. Bloomberg is now a staunch advocate of the hybrid-cab requirement, it actually took several years of prodding by environmental advocates before the city moved to require the use of more fuel-efficient vehicles.

In 2003, the city authorized the taxi commission to sell additional medallions — the valuable licenses that are the exclusive right to operate a cab — provided that at least 9 percent of the new medallions went to hybrids or to cars powered by compressed natural gas.

Over the next two years, the commission failed to approve any hybrids for use as taxis, saying that the existing models did not have sufficient interior room and did not meet other city guidelines. In 2005, the commission began approving hybrid models for use as taxis; there are now 10 types of hybrid and clean-diesel vehicles approved for use as city cabs.

But it was not until last year that the Bloomberg administration fully embraced the hybrids. Mr. Bloomberg announced his broad PlaNYC 2030 program on April 22, 2007. A month later, on May 22, 2007, he announced that the taxicab fleet would become fully hybrid by 2012.

Councilman David Yassky, a Brooklyn Democrat who was one of the original proponents of the push to introduce hybrid vehicles into the taxi fleet, said in a phone interview on Friday, “This ruling is unsound, but even if it stands, there are other ways to get the taxi fleet to go hybrid that can withstand a legal challenge.”

The hybrid-taxi rule has been bitterly fought by taxi owners. Earlier this year, Ron Sherman, president of the Metropolitan Taxicab Board of Trade, said in a statement that “small, light passenger hybrids should not be used as New York City taxicabs, which clock upwards of 100,000 miles a year each and often run 24 hours a day, 7 days a week.” He argued that the Taxi and Limousine Commission “has ignored the laws of physics, which dictates that the larger the vehicle’s interior space, the safer the vehicle’s occupants are in an accident.”

The other plaintiffs were the Midtown Operating Corporation, a garage that leases taxis to more than 800 independent contractors on a double-shifted daily basis; the Sweet Irene Transportation Company, which owns and leases taxis; Ossman Ali of the Bronx, a self-employed independent contractor who leases and drives taxis; and Kevin Healy of Roslyn Heights, N.Y., a frequent taxi passenger.

Judge Crotty said the plaintiffs were likely to succeed on their argument that fuel economy standards were delegated to federal agencies under the Energy Policy and Conservation Act, but he said he did not agree with their argument that the city was trying to improperly regulate tailpipe emissions and supplant the federal role in enforcing the Clean Air Act.

The full text of the mayor’s statement on Friday is below:

We are very disappointed in the decision and we are exploring our appellate options. The decision is not a ruling against hybrid cabs, rather a ruling that archaic Washington regulations are applicable and therefore New York City, and all other cities, are prevented from choosing to create cleaner air and a healthier place to live. The sad irony here is the laws being relied on by the plaintiff, the Clean Air Act and the Energy Policy and Conservation Act, were designed to reduce air pollution and reduce our dependence on foreign oil, which is exactly what moving to fuel efficient cabs will do. The courts are not the only way we can reach our goal of a cleaner fleet of taxi cabs. I’ve instructed the T.L.C. to develop a program with strong incentives for the use of fuel-efficient vehicles and heavy disincentives for use of the inefficient vehicles of a past generation. Additionally, we will be working with our Congressional delegation to produce legislation to update the outdated laws, originally written in the 1970s, to reflect the current realities of environmental stewardship. Greening the taxi fleet is a major priority and we are going to use every mechanism at our disposal to make New York a cleaner, healthier city. Taxis are a part of our public transportation system; they must be part of the solution to air pollution, not a contributing cause of the problem.

Mr. Sherman, the president of the Metropolitan Taxicab Board of Trade, said in a statement:

Millions of people who ride taxicabs and the thousands of drivers, owners and other participants in the New York City taxi industry can breathe a sigh of relief today as this ill-conceived hybrid taxi mandate, which had not properly taken safety, comfort, availability and other issues into account when it was rushed through last year, will not go forward as planned. By granting this injunction, the court has affirmed what numerous groups, associations, state and city legislators, former T.L.C. commissioners, drivers and others have argued for a long time. M.T.B.O.T. has always preferred that this issue be settled out of court. However, after exhaustive efforts at the T.L.C., we were given no choice and were compelled to advocate on behalf of the safety of our passengers and drivers. Now that the court has ruled in our favor, and indeed — in the taxi passengers’ and taxi drivers’ favor — we look forward to working with the Taxi and Limousine Commission and our colleagues in the taxi industry to ensure the continuity of safe, effective, comfortable taxi service for the riding public, ensure a strong and healthy industry and achieve a fuel-efficient, safe, comfortable taxi of tomorrow.

Joshua Nachowitz, policy director of the New York League of Conservation Voters, said in a statement: