America's most controversial writer Gore Vidal has launched the most scathing attack to date on George W Bush's Presidency, calling for an investigation into the events of 9/11 to discover whether the Bush administration deliberately chose not to act on warnings of Al-Qaeda's plans.

Vidal's highly controversial 7000 word polemic titled 'The Enemy Within' - published in the print edition of The Observer today - argues that what he calls a 'Bush junta' used the terrorist attacks as a pretext to enact a pre-existing agenda to invade Afghanistan and crack down on civil liberties at home.

Vidal writes: 'We still don't know by whom we were struck that infamous Tuesday, or for what true purpose. But it is fairly plain to many civil libertarians that 9/11 put paid not only to much of our fragile Bill of Rights but also to our once-envied system of government which had taken a mortal blow the previous year when the Supreme Court did a little dance in 5/4 time and replaced a popularly elected President with the oil and gas Bush-Cheney junta.'

Vidal argues that the real motive for the Afghanistan war was to control the gateway to Eurasia and Central Asia's energy riches. He quotes extensively from a 1997 analysis of the region by Zgibniew Brzezinski, formerly national security adviser to President Carter, in support of this theory. But, Vidal argues, US administrations, both Democrat and Republican, were aware that the American public would resist any war in Afghanistan without a truly massive and widely perceived external threat.

'Osama was chosen on aesthetic grounds to be the frightening logo for our long-contemplated invasion and conquest of Afghanistan ... [because] the administration is convinced that Americans are so simple-minded that they can deal with no scenario more complex than the venerable, lone, crazed killer (this time with zombie helpers) who does evil just for the fun of it 'cause he hates us because we're rich 'n free 'n he's not.' Vidal also attacks the American media's failure to discuss 11 September and its consequences: 'Apparently, "conspiracy stuff" is now shorthand for unspeakable truth.'

'It is an article of faith that there are no conspiracies in American life. Yet, a year or so ago, who would have thought that most of corporate America had been conspiring with accountants to cook their books since - well, at least the bright dawn of the era of Reagan and deregulation.'

At the heart of the essay are questions about the events of 9/11 itself and the two hours after the planes were hijacked. Vidal writes that 'astonished military experts cannot fathom why the government's "automatic standard order of procedure in the event of a hijacking" was not followed'.

These procedures, says Vidal, determine that fighter planes should automatically be sent aloft as soon as a plane has deviated from its flight plan. Presidential authority is not required until a plane is to be shot down. But, on 11 September, no decision to start launching planes was taken until 9.40am, eighty minutes after air controllers first knew that Flight 11 had been hijacked and fifty minutes after the first plane had struck the North Tower.

'By law, the fighters should have been up at around 8.15. If they had, all the hijacked planes might have been diverted and shot down.'

Vidal asks why Bush, as Commander-in-Chief, stayed in a Florida classroom as news of the attacks broke: 'The behaviour of President Bush on 11 September certainly gives rise to not unnatural suspicions.' He also attacks the 'nonchalance' of General Richard B Myers, acting Joint Chief of Staff, in failing to respond until the planes had crashed into the twin towers.

Asking whether these failures to act expeditiously were down to conspiracy, coincidence or error, Vidal notes that incompetence would usually lead to reprimands for those responsible, writing that 'It is interesting how often in our history, when disaster strikes, incompetence is considered a better alibi than .... Well, yes, there are worse things.'

Vidal draws comparisons with another 'day of infamy' in American history, writing that 'The truth about Pearl Harbour is obscured to this day. But it has been much studied. 11 September, it is plain, is never going to be investigated if Bush has anything to say about it.' He quotes CNN reports that Bush personally asked Senate Majority Leader Tom Daschle to limit Congressional investigation of the day itself, ostensibly on grounds of not diverting resources from the anti-terror campaign.

Vidal calls bin Laden an 'Islamic zealot' and 'evil doer' but argues that 'war' cannot be waged on the abstraction of 'terrorism'. He says that 'Every nation knows how - if it has the means and will - to protect itself from thugs of the sort that brought us 9/11 ... You put a price on their heads and hunt them down. In recent years, Italy has been doing that with the Sicilian Mafia; and no-one has suggested bombing Palermo.'

Vidal also highlights the role of American and Pakistani intelligence in creating the fundamentalist terrorist threat: 'Apparently, Pakistan did do it - or some of it' but with American support. "From 1979, the largest covert operation in the history of the CIA was launched in response to the Soviet invasion of Afghanistan ... the CIA covertly trained and sponsored these warriors.'

Vidal also quotes the highly respected defence journal Jane's Defence Weekly on how this support for Islamic fundamentalism continued after the emergence of bin Laden: 'In 1988, with US knowledge, bin Laden created Al-Qaeda (The Base); a conglomerate of quasi-independent Islamic terrorist cells spread across 26 or so countries. Washington turned a blind eye to Al-Qaeda.'

Vidal, 77, and internationally renowned for his award-winning novels and plays, has long been a ferocious, and often isolated, critic of the Bush administration at home and abroad. He now lives in Italy. In Vidal's most recent book, The Last Empire, he argued that 'Americans have no idea of the extent of their government's mischief ... the number of military strikes we have made unprovoked, against other countries, since 1947 is more than 250.'