The Labor Department has warned the AFL-CIO not to attempt to use pension funds as a cudgel in the battle over President Bush’s plan to add private accounts to the Social Security system.

In a letter, a federal official who oversees the regulation of benefit plans told the labor federation that it would be illegal for any retirement plan to hire or fire a financial adviser based on the firm’s views on Social Security reform.

“It would be unlawful for a plan fiduciary to review the plan’s service providers based, not upon the quality and expense of their services, but rather upon their views on Social Security or any other broad area of public policy,” a Labor Department deputy assistant secretary, Alan Lebowitz, wrote in the letter, dated May 3. “Plan fiduciaries may not increase expenses, sacrifice investment returns, or reduce the security of plan benefits in order to promote collateral goals.”

An attorney for the AFL-CIO, Damon Silvers, said the labor federation agrees with the department’s legal analysis and is distributing the letter to pension funds. “The way the law is laid out, we felt, is the way we’ve been doing it already,” he said. “Pension funds ought not to be funding political campaigns, and we agree with that, and they’re not.”

Mr. Silvers noted that trustees run union pension plans, not the labor federation. He said that the plans could inform members about the impact of various proposals without running afoul of the law.

Mr. Lebowitz’s letter noted press accounts that indicated the labor movement planned to pull pension money out of banks and investment firms that backed Mr. Bush’s proposal.

“We have no intention of letting any of these companies get away with this while they manage our workers’ funds,” an AFL-CIO lobbyist, Gerald Shea, said in January to Roll Call, a newspaper that covers Congress.

Soon thereafter, two brokerage firms, Edward Jones and Waddell Reed, withdrew from an industry group that supports Mr. Bush’s plan, the Alliance for Worker Retirement Security.

“This letter suggests that union leaders may have violated federal law, and I expect the department to follow up on this matter and investigate it vigorously,” Rep. John Boehner of Ohio said in a statement yesterday.

Rep. George Miller, a Democrat of California, said no investigation was warranted because the Labor Department’s letter contained “absolutely no suggestion whatsoever that anyone at the AFL-CIO has violated any law.”