TV news reports in America that showed President George Bush getting a standing ovation from potential voters have been exposed as fake, it has emerged.

The US government admitted it paid actors to pose as journalists in video news releases sent to TV stations intending to convey support for new laws about health benefits.

Investigators are examining the film segments, in which actors pretending to be journalists praise the benefits of the new law passed last year by President Bush, to see if they could be construed as propaganda.

Two of the films are signed off by "Karen Ryan", who was an actor hired to read a script prepared by the government, according to production company Home Front Communications.

Another video, intended for Hispanic viewers, shows a government official being interviewed in Spanish by a actor posing as a reporter with the name "Alberto Garcia".

One segment shows a pharmacist telling an elderly customer the new law "helps you better afford your medications".

"It sounds like a good idea," the customer says, to which the pharmacist replies, "A very good idea."

And in some scenes President Bush is shown receiving a standing ovation from a crowd cheering him as he signed the Medicare law, which is designed to help elderly people with prescriptions.

The government also prepared scripts to be used by news anchors. "In December, President Bush signed into law the first-ever prescription drug benefit for people with Medicare," the script reads.

"Since then, there have been a lot of questions about how the law will help older Americans and people with disabilities. Reporter Karen Ryan helps sort through the details." The "reporter" then explains the benefits of the new law.

Lawyers from the investigative arm of Congress discovered the tapes as part of an investigation into federal money that was used to publicise the new law.

They will be keen to ascertain whether the government might have misled viewers by failing to reveal the source of the videos, which were broadcast in Oklahoma, Louisiana and other states.

"Video news releases" of this sort have been used in the US since the 1980s, but the way they blur the lines between news and advertising troubles many media experts and campaigners.

The government defended the videos, which Democrats described as "disturbing". "The use of video news releases is a common, routine practice in government and the private sector," a health department spokesman told the New York Times.

VNRs are also used in Europe but a furore surrounding a Greenpeace video package about its campaign to prevent the dumping of Shell's Brent Spar oil platform sent to British broadcasters some years ago led to new rules clamping down on their use.

Greenpeace's sophisticated media offensive - including the provision of emotive film footage of its occupation of the platform - resulted in one-dimensional coverage by BBC and ITN, news chiefs admitted at the time.

Guidelines were subsequently drawn up to label video news releases as such - a category which the regular Osama bin Laden videos now fall.