Without seeking legislative approval, the Spitzer administration has quietly ordered charter schools to start paying union wages on all construction, repair, and maintenance projects.

Handed down by an administration that has encouraged the growth of charter schools in New York, the new policy came as a surprise to charter school operators and advocates, who say it will force them to spend as much as 30% more on building costs and divert classroom expenditures.

Several charter school groups have filed suit against the state’s Department of Labor to overturn the wage mandate, which went into effect September 20, contending that education law exempts the schools from paying union wages and that the labor department, in mandating the change, usurped the authority of the state Legislature, whose members have tried but failed to pass a similar measure.

Labor department officials say the agency is enforcing a policy that was established in 2000 in a formal opinion issued by the attorney general’s office headed at the time by Governor Spitzer.

While a labor department official insisted that Mr. Spitzer stood behind his agency’s policy, the governor, who signed legislation this year raising a cap on the number of charter schools to 200 from 100, appears to be taking a more equivocal stance.

A spokesman for Mr. Spitzer said the labor department “has the authority” to set the mandate, but he stopped short of endorsing it, saying it will be up to the courts to “make the final decision on the legal question of whether prevailing wages must be paid on charter school construction projects.”

A spokesman for the labor department, Leo Rosales, said, “This is part of labor department policy, so it is something that has support of the administration.”

Prevailing wage law requires that laborers be paid a minimum hourly rate on public works projects. The rate, which is set by the labor department, generally corresponds to local union wages.

A boilermaker working on a public construction project in New York City, for instance, must be paid at least $44.09 an hour, according to the prevailing wage schedule posted on the labor department’s Web site. The rate increases to $176.36 an hour — four times the regular wage — for work done on Labor Day, not including supplemental benefits.

At the start of the school year, the labor department’s commissioner, Patricia Smith, who enforced state labor laws while Mr. Spitzer was attorney general, alerted charter schools in a letter that they were now subject to the state’s prevailing wage mandates stipulated in state labor law.

Ms. Smith, who helped write the 2000 opinion as head of the office’s labor bureau, said in the letter that construction work at charter schools constitutes a “public work project” in part because the schools “serve a valuable public purpose” and because they owe their existence to charters issued by state entities.

“The letter came out of nowhere, and people are looking to find out whether it’s going to be enforced or whether it’s the Spitzer administration’s belief that this is how the law is going to be interpreted,” the executive director of Democrats for Education Reform in New York City, Joseph Williams, said. “People are not looking to jump to quick conclusions. They are looking to see from Spitzer how committed he is to this concept.”

Labor department officials say they dispute the claim by charter schools that the mandate will lead to inflated building costs. “There is no factual evidence out there to support that. Prevailing wage isn’t the highest wage out there. It’s comparable to other wages,” Mr. Rosales said.

Charter schools, which are publicly funded but are independent of local school district bureaucracy, are exempted from many of the statutes that govern regular public schools, excluding those having to do with health, safety, and civil rights.

Advocates say the exemptions are a critical advantage for the schools, which are excluded from the almost $2 billion in annual state building aid that flows to regular public schools.

About half of the 60 charter schools in New York City have limited capital costs because they share space with public school facilities, while several other schools in the city receive capital funding from Mayor Bloomberg’s education department.

The remaining schools that rent space — about 50 in and outside of New York City — pay for construction, renovation, and maintenance costs out of their own budgets.

The Bureau of Public Work, which administers the prevailing laws and inspects construction sites, has not cited a charter school since the labor department changed its policy.