Swiss researchers say they are 95 per cent certain that a recent audio tape attributed to the Saudi dissident, Osama bin Laden, is not genuine.
The Lausanne-based Dalle Molle Institute for Perceptual Artificial Intelligence (IDIAP) claims it was recorded by an impostor.
The review of the tape, which was first aired on November 12 by the Arabic television network, Al-Jazeera, was commissioned by the French television channel, France 2.
However, having compared the most recent tape with 20 previous recordings attributed to bin Laden, the IDIAP claims the voice is not that of the al-Qaeda leader.
Professor Hervé Boulard, the institute's director, told France 2 that he was 95 per cent certain that "it has not been recorded by bin Laden".
The IDIAP, which is affiliated to the Federal Institute of Technology in Lausanne and the University of Geneva, said the risk of error was just five per cent.
The voice on the tape praises recent attacks against Western targets including the siege of a Moscow theatre by Chechen rebels, the bombing of a French oil tanker off the coast of Yemen, and the Bali bombing.
The speaker also warns America's allies - particularly Britain, France, Italy, Canada, Germany and Australia - that they will also be targeted if they continue to back the US.
At the time of the tape's appearance, officials in the United States concluded that it was "probably" bin Laden speaking. They said it was the clearest evidence yet that bin Laden had survived the US-led bombing in Afghanistan.
The Swiss findings, however, are in direct contrast to voice recognition tests commissioned by the weekly French news magazine, "L'Express".
The magazine asked two experts to examine the tape: Bernard Gautheron, director of the phonetic testing laboratory at the University of Paris's Institute of Linguistics and Phonetics; and Olivier Fiani, a translator and expert on Middle East affairs.
Gautheron concluded that there was a "very strong probability" that the tape was authentic.
In an interview with swissinfo, he cast doubt on whether the computers used by the IDIAP had been subtle enough to pick up all the known nuances of bin Laden's speech pattern, including his use of language.
"Osama bin Laden's speech is characterised by a certain number of tics," he said. "Computers do not take into account these elements which are nonetheless very important."
swissinfo with agencies
The Swiss institute compared the tape with 20 previous recordings of bin Laden.
A recent French study found opposite conclusion saying voice was probably authentic.
The tape of bin Laden praising Bali attacks was first aired in November.