Movie studios backing the rival HD DVD and Blu-ray Disc optical formats have until tomorrow to tell the European Commission about how they chose which of the two to back and why.

The move to request information from the studios follows the EC's demand, made a year ago, that hardware vendors provide similar details.

The EC probe is not a formal anti-trust investigation - it's still at a "fact-finding" stage, a spokesman told the Reuters news agency - suggesting that reports in the press that the Commission is seeking reams of phone records and emails are overblown.

The Commission is, however, keen to ensure that consumer interests are not being harmed by hardware makers and content providers' decisions to support one format and not the other.

HD DVD is being driven primarily by Toshiba, with support from studios Universal, Warner and Paramount, though the last two are also backing Blu-ray Disc. Sony and 20th Century Fox only release HD content on Blu-ray, a format promoted by Sony, Samsung, Philips and others.

Toshiba has already announced that it will ship HD DVD drives as standard in all its laptops from 2008, while video and games rental chain Blockbuster has put its weight strongly behind Blu-ray in America. Warner Home Video is set to release an HD DVD/Blu-ray disc in January 2008.