Dutch electronics giant Philips has said it will "vigorously oppose" any suggestion that it and Korean firm LG fixed the prices of LCD flat screens.
Philips' comments came after European Commission competition regulators sent it a "statement of objections" to formally outline their suspicions.
Brussels suspects Philips and LG may have been part of a wider cartel.
The investigation relates to a jointly-owned Philips and LG business - LG Display - which was set up in 1999.
Philips has subsequently sold its share in the unit.
Issuing a statement of objections is the first formal step in European Union anti-competition investigations, under which the Commission first informs the parties concerned about the objections raised against them.
It does not prejudice the outcome of an investigation, and firms can reply to the objections.
Last November, LG Display pleaded guilty to LCD price-fixing in the US, paying $400m (£249m) following a parallel investigation by US competition regulators.
Japan's Sharp and Taiwan's Chunghwa Picture Tubes were also fined by US authorities in that case.
Brussels has not said which other firms are involved in its investigation.
The news comes on the same day that Philips reported a 94% drop in second quarter profits due to a big decline in sales.