The move confirms the growing belief among studios that a format war is now inevitable and that both sides will need to be supported until one becomes dominant.
Warner's decision will come as a major blow to the HD-DVD camp as the entertainment giant has the largest film distribution network in the world and owns New Line Cinema and HBO, which produces series like Sex and the City and The Sopranos.
Warner had been due to start releasing titles in the HD-DVD format by the end of this year and has been working with Microsoft to integrate DRM technology onto the discs. The DRM capabilities of HD-DVD were one of the key reasons cited by Warner for its initial backing of the format.
"HD-DVD innovations include higher resolution video and audio available on a suite of disc capacities adaptable for longer or shorter programmes," said Chris Cookson, chief technology officer at Warner Brothers Entertainment last year. " HD DVD is also supported by advanced security technology and proven manufacturability."
A full blown standards war now looks impossible to avoid and may last two years, according to analysts. The two sides recently abandoned talks to find a common standard, although Samsung has said it will make a dual format media player.
Intel and Microsoft, as well as the two biggest Chinese hardware manufacturers, are supporting HD DVD, but Blu-Ray has some impressive names behind it including Apple, Universal and Lion's Gate Entertainment.
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