The European Commission has launched an informal investigation into the market for next-generation DVDs, and has set a deadline of 6 July for submissions.
Information has been requested from manufacturers and content providers concerning the decision of which high-definition DVD format to support.
European Commission spokesman Jonathan Todd told Reuters: "We have sent a request for information to a number of studios concerning possible discrimination against one or other DVD format.
"We are at the stage of basic fact-finding, and have not yet opened a formal investigation."
The Commission is particularly interested in why so many studios went for the Blu-ray format exclusively.
Only one Hollywood studio, Universal, has an exclusive agreement with HD-DVD, while some are producing films for both formats and five exclusively for Blu-ray.
There have been no complaints of anti-competitive behaviour but it is understood that the HD-DVD camp has been lobbying behind the scenes for an investigation.
The Commission has asked for briefing documents, emails and any phone records relating to the issue.
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