When it emerged that Gannon was also linked to gay prostitution websites and might be a gay prostitute himself, the scandal as to how he was allowed daily access to the White House grew even murkier. The American media is now being forced to confront the possibility that Gannon, whose real name is James Guckert, was simply a Republican plant, used by officials, including President George W Bush, to ask easy questions in difficult press conferences. 'The idea of having a mole in the White House press corp is amazing, but that's what it looks like,' said Jack Lule, a journalism professor at Lehigh University.

But the Gannon affair, which has shocked much of America's political establishment, is just the latest scandal in the media establishment. Newspapers including the New York Times and USA Today have been hit by plagiarism and forgery scandals. Other papers and television stations have been consumed with a soul-searching inquest into how they were misled about non-existent Iraqi weapons programmes. Added to that is growing evidence of a White House campaign to bypass or control the media in its everyday presentation of government policy , which included paying one journalist hundreds of thousands of dollars to promote its policies.

Last week a federal watchdog warned the Bush administration that any video news releases must state that the government is the source. Twice in two years, government departments have been accused of distributing fake news packages, using actors as journalists.

On the internet, the mainstream media is derided and scorned. One question is dominating US newsrooms and television studios: ignored, scandalised and now corrupted, just what is America's mainstream media for anymore?

The extent of the Bush White House's command and control of the press corps is often revealed in the seemingly innocuous White House pool reports. These are dispatches dutifully filed by a correspondent assigned to travel with Bush and contain little but lists of endless meetings, meals eaten and clothes worn. But no detail is too small to be ignored by Bush's ever-watchful press handlers. One report, on 13 August 2004, contained a remark from Bush that it was a 'good question' as to who to support if Iraq's soccer team played the United States in the Olympics. Officials scurried to 'correct' it. 'To clear up any possible misconception ... the president would of course support the American soccer team in any hypothetical game with Iraq,' a new report said. 'The initial report should have done more to reflect the exchange was mainly in jest.'

Such micromanagement has been a hallmark of the Bush White House and its all-powerful policy guru, Karl Rove. Added to that has been what appears to be a concerted effort to subvert the mainstream media.

Administration officials were recently revealed to have paid three senior journalists to promote or design policies. More than $240,000 of taxpayers' cash was paid to black pundit Armstrong Williams to push the agenda of Bush's education department. Critics were blunt in their assessment of what Armstrong's contract with the government meant. 'It is propaganda,' said Melanie Sloan of watchdog group Citizens for Responsibility and Ethics.

At the same time, Bush has held fewer Washington press conferences than any of his modern predecessors, while courting local media, such as small city newspapers, which are perceived as easier to steamroll. During last year's election campaign Bush avoided interviews with leading newspapers, such as the Washington Post , but frequently invited reporters from smaller swing state publications to speak with him on Air Force One. Vice-president Dick Cheney took the strategy one step further and banned New York Times reporters from travelling with him.

The media has not helped its own case. First, New York Times reporter Jayson Blair was found to have plagiarised numerous stories. The incident cost Blair his job, forced the editor to resign and was the subject of fevered Manhattan dinner party chatter for months. Then USA Today 's top foreign reporter, Jack Kelley, was discovered to have fabricated stories from around the world and invented interviews and witnesses from Cuba to Jerusalem.

Right-wing media ratcheted up the long-standing conservative complaint that the media is dominated by liberal publications. Though many journalism experts deny that is the case, the image has settled in the American consciousness, forcing newspapers, magazines and television stations to go out of their way to prove they are not liberal. 'We have a conservative media and also a mainstream media, which is also now fairly conservative because it has been forced to deny being liberal,' said Lule.

The Gannon case is a prime illustration. If, during the Clinton administration, a fake reporter from a Democrat front organisation, using a false name, had been exposed as attending White House press conferences it would have been a national scandal. If he had then been shown to be a gay prostitute, the scandal could have threatened a Democrat presidency. With 'Gannon' and Bush there has been no such outcry. The mainstream media has approached the story warily, while right-wing organisations such as Fox News have largely ignored it.

That has created a vacuum in the US media. It is a space being filled by 'bloggers' from both left and right who write personal journals, or weblogs, on the internet. It is here that the real media battles are now being fought. The internet has become a sort of Fifth Estate as the Fourth Estate of the mainstream media has slid toward irrelevance. The groundwork was done mainly by the right. Internet gossip hound Matt Drudge, whose Drudge Report is a key source for every American political journalist, struck the first blow with his breaking of the Monica Lewinsky affair.

Since then a plethora of right wing blogs have sprung up. Unlike Britain, where political blogs are barely part of the debate, internet sites in America are seen as a vital political tool. Conservative bloggers have taken two big scalps recently. Last year bloggers questioned the veracity of a CBS news report on Bush's National Guard service. They dumped enough doubt on the story to cause four CBS reporters to lose their jobs, tarnish the reputation of legendary anchor Dan Rather and insure that the substance of the CBS story - whether Bush fulfilled his service - never emerged as an election issue.

Last week, CNN's chief news executive, Eason Jordan, resigned after an internet campaign prompted by his claim that American soldiers targeted journalists in Iraq. Though Jordan said that his remarks had been misinterpreted, the bloggers' revenge was so vehement he ended his 23-year CNN career. One anti-Jordan website, Easongate.com, crowed openly when he quit: 'To every reader, commentator, e-mailer and blogger that committed to this cause, thank you.'

The left has also had victories. It was not the mainstream media that exposed Gannon, but left-wing website Media Matters for America which enlisted other liberal bloggers to help. All the significant breaks in the story emerged online, forcing Gannon to resign, reveal his real name and go into hiding.

Some commentators see the emergence of blogging as a media force as a liberating phenomenon. Unlike the mainstream media, blogging is cheap, easy and open to anyone regardless of qualification or background or money. 'Blogging gives a voice to those who were previously silent,' said Ananda Mitra, a communications professor at Wake Forest University.

Others see it as part of the trend towards partisan journalism. Spearheaded by the nakedly right-wing Fox News, journalism in America has come to resemble a political shouting match rather than any form of debate of the issues. But with soaring viewership, Fox has emerged as one of the most powerful forces in the media landscape. Other networks, such as CNN and MSNBC, have sought to copy Fox's personality-led and opinion-based news.

The media is in the midst of a transformation which the Bush administration is keen to foster. They have discovered that a partisan and atomised media can be controlled, manipulated and used to an unprecedented degree.

It is a lesson that liberals are also learning. In answer to the talk radio of Rush Limbaugh - one of America's most popular and conservative commentators - liberal groups have set up Air America. Defying the critics, it has established itself as a left-wing radio network every bit as ruthless in skewering its opponents' points of view as its right-wing equivalents. In answer to right-wing television, former presidential candidate Al Gore is rumoured to be seeking backers to finance a liberal television network. Now both sides are equally ready and willing to use any means necessary to tear the other apart. The old-fashioned mainstream media is disappearing. 'Once that pattern is put in place, it is going to be hard to break,' said Lule.

How the media shot themselves in the foot
A series of scandals have not helped the American media's reputation and its struggle for independence.

New York Times

Reporter Jayson Blair was fired and the newspaper's editor forced to resign after Blair was found to have plagiarised numerous stories.

USA Today

Foreign reporter Jack Kelly was discovered to have invented stories, interviews and witnesses from around the world.

CBS

Four reporters lost their jobs and the reputation of legendary anchor Dan Rather was tarnished after doubts were cast on a news report of Bush's National Guard Service.

CNN

Chief news executive Eason Jordan resigned his 23-year career after he claimed that American soldiers had deliberately targeted journalists in Iraq.