WASHINGTON, Feb. 18 - The comptroller general has issued a blanket warning that reminds federal agencies they may not produce newscasts promoting administration policies without clearly stating that the government itself is the source.

Twice in the last two years, agencies of the federal government have been caught distributing prepackaged television programs that used paid spokesmen acting as newscasters and, in violation of federal law, failed to disclose the administration's role in developing and financing them.

And those were not isolated incidents, David M. Walker, the comptroller general, said in a letter dated Thursday that put all agency heads on notice about the practice.

In fact, it has become increasingly common for federal agencies to adopt the public relations tactic of producing "video news releases" that look indistinguishable from authentic newscasts and, as ready-made and cost-free reports, are sometimes picked up by local news programs. It is illegal for the government to produce or distribute such publicity material domestically without disclosing its own role.

Mr. Walker, who as comptroller general is chief of the Government Accountability Office, Congress's investigative arm, said in his letter: "While agencies generally have the right to disseminate information about their policies and activities, agencies may not use appropriated funds to produce or distribute prepackaged news stories intended to be viewed by television audiences that conceal or do not clearly identify for the television viewing audience that the agency was the source of those materials."

"It is not enough," he added, "that the contents of an agency's communication may be unobjectionable."

Mr. Walker's letter was made available late Friday afternoon by Democrats on Capitol Hill. Asked for a response Friday night, the White House had no immediate comment.

The two best-known cases of such video news releases - one concerning the new Medicare law, the other an antidrug campaign by the Bush administration - drew sharp rebukes from the G.A.O. after separate investigations last year found that the agencies involved had violated the law.

Those cases were followed by disclosures that the government had paid at least one conservative commentator, Armstrong Williams, to promote the administration's No Child Left Behind education measure and had put two other conservative writers on the federal payroll to help develop programs. These episodes have prompted calls from Democrats for stricter oversight of the administration's publicity practices, which have cost millions of dollars of federal revenue.

In the Medicare case, a video made in the style of a newscast featured a spokeswoman named Karen Ryan who claimed to be reporting from Washington on Medicare law changes strongly backed by the administration but opposed by many Democrats, who consider them a windfall for the pharmaceutical and insurance industries. In part of one script, she said that "all people with Medicare will be able to get coverage that will lower their prescription drug spending."

Often there is an intermediary in the process: a public relations firm hired by a government agency to produce a polished video and direct other aspects of a publicity drive.

One centrally involved firm is Ketchum, a giant in the public relations industry whose representatives arranged for both the Medicare video and the contract with Mr. Williams, a pact that is now under investigation by three government agencies. Ketchum has received $97 million in government public relations contracts since 2001.

The G.A.O. letter did not caution agencies to curtail their publicity practices, telling them simply to adhere to disclosure requirements.

"Prepackaged news stories," Mr. Walker wrote, "can be utilized without violating the law, so long as there is clear disclosure to the television viewing audience that this material was prepared by or in cooperation with the government department or agency."

But Democrats said they hoped the letter would lead to tougher scrutiny of what they describe as an aggressive publicity machine within the administration.

"The G.A.O. is sending a clear message to the Bush administration: shut down the propaganda mill," Senator Frank R. Lautenberg of New Jersey said in a statement on Friday. "The G.A.O. is simply telling the White House to stop manipulating media, stop paying journalists and be straight with the American people."

Commentary:
The Bush White House has to be warned not to create fake news stories. Good grief, it's become that bad. How many laws does Bush get to break before he's removed from office? Will republicans in Congress ever have our respect when they allow this president to break so many of our laws? With each passing day (and year), the Clinton impeachment is seen for what it really was, a political abuse of power by the republican party.