The White House yesterday came under pressure to give more details of the video which purports to show Osama bin Laden admitting his part in the September 11 attacks.
There was growing doubt in the Muslim world about the authenticity of the film while special effects experts said computer technology made it possible to fake such a video. Unless the US gives more information about how the tape was found or provides more technological details about it, doubts are bound to linger.
On the face of it the video is the "smoking gun" that proves Bin Laden's part in the murder of more than 3,000 people in the World Trade Centre and Pentagon attacks. President Bush yesterday called it a "devastating declaration of guilt for this evil person".
The 40-minute poor-quality tape, apparently shot with a camcorder, shows Bin Laden telling a visiting Muslim cleric details of the planning for the attack and his delight in the carnage.
According to US officials the tape was found in a house in Jalalabad, eastern Afghanistan, and handed to the Pentagon by an unnamed person or group. Officials say Mr Bush first watched the tape in November but release was delayed until it could be authenticated. Independent translators were used to make sure the US could not be accused of twisting the words of the men on the film.
But for many the explanation is too convenient. Some opponents of the war theorise that the Bin Laden in the film was a lookalike, others claim images of him had been manipulated.
It was also pointed out that it was surprising that a man with the ability to organise the attacks on America would be naive enough to confess on tape. And some observers point out that Bin Laden appears to be wearing a ring on his right hand. In previous film of Bin Laden released by him, he has worn no jewellery apart from a watch.
Riaz Durrani, a spokesman for Jamiat Ulema-e-Islam, which spearheaded pro-Taliban rallies in Pakistan, said: "This videotape is not authentic. The Americans made it up after failing to get any evidence against Osama."
Legal experts in the US said that prosecutors seeking to bring Bin Laden to justice would certainly be keen to produce the tape but might struggle to prove its authenticity.
Henry Hingson, a former president of the national association of criminal defence lawyers, said: "In this day and age of digital wizardry, many things can be done to alter its veracity."
On the other hand it would be foolish to fake a video confession, knowing that if Bin Laden is ever tried his defence team will have experts pore over the video.
Sean Broughton, director of the London-based production company Smoke and Mirrors and one of Britain's leading experts on visual effects, said it would be relatively easy for a skilled professional to fake a video of Bin Laden.
The first step would be to transfer images shot on videotape on to film tape. Distortion or "noise" and graininess would be removed. A "morphing package" would then be used to manipulate the image on a computer screen.
Using such a package it is possible to alter the subject's mouth and expressions to fit in with whatever soundtrack is desired. The final step is to put the "noise" and graininess back on and transfer the doctored images on to videotape.
In a recent advert that Smoke and Mirrors made for a US insurance company, the technique was used to place Bill Clinton's head on an actor's body for comic effect.
Mr Broughton said that while it would be relatively easy to fake a Bin Laden video, to fool the top experts was much more difficult. "There are perhaps 20 people in America who would be good enough to fool everybody. To find someone that good and make sure they kept quiet would probably be pretty difficult."
Bob Crabtree, editor of the magazine Computer Video, said it was impossible to judge whether the video was a fake without more details of its source. "The US seems simply to have asked the world to trust them that it is genuine."
Mr Bush said it was "preposterous for anybody to think this tape was doctored".
He added: "Those who contend it's a farce or a fake are hoping for the best about an evil man. This is Bin Laden unedited. This is... the Bin Laden who murdered the people. This is a man who sent innocent people to their death."
The foreign secretary, Jack Straw, insisted there was "no doubt it is the real thing".