Film studios have been dragged into the European commission's informal investigation into the high-resolution DVD market.

The commission is concerned that the companies behind the two rival next-generation DVD technologies, Blu-ray and HD-DVD, may be restricting competition by asking studios to release new films on only one format.

A spokeswoman confirmed today that the movie studios have been asked to supply information about any restrictions they have agreed to. The commission appears to be particularly interested in the Blu-ray format.

"We want more information about whether the studios have arrangements with the Blu-ray format, and if so what those arrangements are," explained a spokeswoman for competition commissioner Neelie Kroes.

The requests were sent out in mid-June, and the studios have until July 6 to respond.

Blu-ray and HD-DVD have been locked in a format war that is reminiscent of the infamous Betamax-VHS video recorder battles of the 1980s.

Both technologies can store much more information than a standard DVD, which results in sharper picture and sound quality. But there are very few next-generation DVD players that support both formats.

Sony is one of the biggest Blu-ray supporters, and has used the technology in its PlayStation3 games console. HD DVD's backers include Toshiba and Microsoft.

According to the Wall Street Journal, five major movie studios are exclusively supporting Blu-ray – Sony Pictures Entertainment, Walt Disney, Twentieth Century Fox, Lions Gate Entertainment and MGM. Just one, Universal Pictures, is releasing films only in the HD DVD format.

The EC could launch a formal investigation if it believes either side is unfairly restricting competition.