The US president planned to bomb the Qatar-based channel - that was the remarkable claim made in a top-secret memo. Why is the world's most powerful man so worried about a TV station?

It was an ambush. As Britain woke up yesterday morning to snowstorms and arctic temperatures, the Attorney General was preparing to be questioned on Radio Four's Today programme over government plans to scrap jury trials in complex fraud cases.

Instead, as the interview with John Humphrys ground to an end shortly after 8.30am, the urbane Lord Goldsmith found himself explaining why he had warned national newspapers not to reveal the contents of a top secret memo detailing a lengthy conversation between the Prime Minister and President George Bush over the direction of the war in Iraq.

'I wasn't seeking to gag newspapers; what I said to newspapers was you need to take legal advice,' Goldsmith insisted as an increasingly irritable Humphrys accused him of trying to silence the media for political expediency. 'It is not being used to save the embarrassment of a politician,' Goldsmith persevered. 'That is completely not the case at all.'

It is unlikely this will be the last time Goldsmith will be asked about the memo, which first emerged in the offices of a little known Northamptonshire MP in June last year, and has metamorphosed into a major diplomatic incident.

The status of the now infamous five-page document concerning the meeting between Bush and Blair, on 16 April last year, has already reached mythic proportions among bloggers on the internet. It is the smoking gun to end all smoking guns, claim conspiracy theorists, who believe it details everything from an agreed date to pull the troops out, to plans to take the one-time rebel stronghold of Fallujah.

The one indisputable fact, though, is that part of the memo - 10 lines to be precise - concerns a conversation between Bush and Blair regarding Al Jazeera, the Arabic satellite television station that the US accuses of being a mouthpiece for al-Qaeda. According to those familiar with the memo's contents, Bush floated the idea of bombing the Qatar-based station. The Daily Mirror, which ran the story last Tuesday, claimed the Prime Minister talked Bush out of the plan.

As they attempted damage limitation last week, government officials suggested Bush's comments were nothing more than a joke. It was preposterous to suggest Bush would countenance such an idea, the officials said. The White House described the allegations as 'unfathomable' although according to those who have seen the memo 'there is no question Bush was serious.'

But whether said in jest or not, the memo reveals Bush's profound obsession with Al Jazeera, an obsession that stretches from stucco-clad government offices in Washington to the tin huts located behind the razor wire in Guantanamo Bay. Why is the most powerful man in the world worried about a 24-hour news organisation?

Salah Hassan, an Al Jazeera camerman, was arrested by US forces in November 2003, while filming the aftermath of an attack on a US convoy near the city of Baquba. Following his arrest he was surprised to discover he had been trailed by US troops for weeks and had been secretly photographed at the scene of other attacks. When he was interrogated, he was accused of having prior knowledge of attacks on coalition forces.

At the heart of the accusation is the fundamental tension between journalists - largely Arab reporters catering for an Arab audience - who say they are anxious to cover the story from both sides, and a United States that regards reporting on some aspects of the insurgency as tantamount to collaboration with terrorism. None of which would matter much were into not for the ferocious tenacity and professionalism of Al Jazeera, factors which have made the station an international phenomenon.

Most gallingly for the US, its reporters have told a story that Washington either disagrees with or would rather remain untold: that the kind of war America is prosecuting in Iraq is messy and heavy handed; that civilians are too often the victims, and that the insurgents are not shadowy sinister figures but ordinary men with more support than politicians would like to acknowledge.

As a result Al Jazeera has seen itself under almost constant attack by a White House whose instinct has been to control the media since the war in Afghanistan. The US military has harassed its reporters. Its offices in Baghdad and Kabul have both been bombed by the US and reporters have been detained, threatened and abused.

The reason, perhaps, is not so difficult to fathom. In the aftermath of the 9/11 attacks, the station, which now enjoys a viewership of some 50 million, began broadcasting a series of messages from Osama bin Laden. It was a remarkable scoop, but one for which the station would pay heavy consequences, convincing the US that Al Jazeera was, at the very least, infiltrated by Al Qaeda. Was bin Laden using the broadcasts to send secret messages to his followers?

By the April of 2004 - and the first battle of Fallujah - US official loathing of the channel had reached a tipping point. Its focus was the figure of Ahmed Mansour, who reported from the city.

According to Sami Muhyideen al-Hajj, an Al Jazeera camerman arrested in Afgahnistan in 2001 and detained in Guantanamo Bay, US interrogators are obsessed with the idea of Al Qaeda infiltration of the channel and asked about Mansour over 100 times.

On separate occasions the reporter and producer has been accused of membership of the radical Muslim Brotherhood (which he denies) and forming 'improper' relationships with mujahedeen leaders when he covered the Russian wars in Afghanistan and Chechnya.

'I don't know why they would ask about me over a hundred times,' Mansour told The Observer last week. 'But the American authorities did not hide their extreme annoyance and fury as a result of my coverage of the first Fallujah campaign in April of 2004. My sole crime was broadcasting the reality of a war I was witnessing.'

Mansour has become something of a cause celebre, lionised almost as much in the West as he is the Middle East. And the fact remains, despite the plethora of accusations against Al Jazeera by its political enemies, so far only one case of Al Qaeda collaboration has been brought to court - that of Taysir Allouni in Spain. A celebrated war correspondent who has interviewed bin Laden, Allouni was jailed for seven years after being cleared of the main charges in a case some claim was more political than evidential.

It was a Friday afternoon in early June last year when a five-page document lying in his pigeon-hole in his constituency office caught the attention of Tony Clarke, the then Labour MP for Northampton South. It was immediately clear to him this was no ordinary party political memo. As he began to read the document, it became apparent it contained extraordinary details of a discussion between the British Prime Minister and the US President during Tony Blair's visit to the White House the previous April.

Written by a Blair aide who accompanied the Prime Minister to Washington it was headed 'top secret'. It is understood that on the five pages there were details of troop deployments and movements. Lurking within the pages were also frank discussions over the US assault on Fallujah. It was clear from the tone of the memo that Blair was far from happy at the tactics used by American forces.

Then, within a few short lines, came the bombshell: documentary evidence the US president had openly talked about bombing Al Jazeera. Clarke, who had voted against military intervention in Iraq believed he had no choice but to call Downing Street and reveal what he had been sent. The next day officers from Special Branch interviewed him at his home.

Clarke's trusted 42-year-old political researcher, Leo O'Connor, was also questioned. Detectives investigating the leak searched Clarke's offices in the House of Commons. It was not long before they had set their sights on the potential leaker as David Keogh, a 49-year old civil servant who had been seconded from the Foreign Office to the Cabinet Office.

Keogh lives alone in Northampton, not far from O'Connor, and was a member of the Labour Party and an occasional member of a dining club that Clarke and other senior members of the local Labour Party attended.

He was arrested on 1 September last year and 10 days ago was charged with sending the document to O'Connor some time between 16 April and 28 May 2004 in breach of the Official Secrets Act. O'Connor, in turn, has been charged with receiving the document under section 5 of the Official Secrets Act.

Details of the Foreign Office's deep misgivings over the way the US was prosecuting the war had already been reported on 23 May last year in the Sunday Times , which had obtained another leaked document. Back then there was no desire by the Attorney General to pick a fight with Fleet Street and threaten it with the Official Secrets Act. Some suspect the difference this time is down to the fact the Al-Jazeera memo has deeply embarrassed the Bush administration.

Keogh and O'Connor are due to appear at Bow Street magistrates' court, south west London, this Tuesday. This initial hearing is likely to be held behind closed doors while the defence and prosecution teams fire their opening salvos. Crucial to the case will be whether the judge believes any of the case can be heard in public.

Legal experts draw comparisons with the case of Katherine Gunn, the former GCHQ employee who alerted the world to the United Nations bugging scandal. The case against Gunn collapsed over fears British intelligence secrets would be revealed in open court and lead to questions over the legality of the war.

The conclusion must be the government's legal advisers believe it won't be forced into such a humiliating climbdown when the case goes to court. This time around the government isn't going to blink.

An arab rival to the BBC

The television station was set up in 1996 with $150 million from the Emir of Qatar.

It was accused of being al-Qaeda's mouthpiece when it broadcast a message from Osama bin Laden praising the 9/11 hijackers.

Many Muslim viewers criticise it for being too pro-Western and giving airtime to Israeli officials.

It now rivals the BBC, with a worldwide audience of 50 million. An English-language satellite service will launch in March 2006.

In 2004 Index on Censorship praised its 'courage in circumventing censorship and contributing to the free exchange of information in the Arab world'.

