An investigation into possible anti-competitive behaviour in the flat-panel display market has widened further, with Sharp joining LG Philips and Samsung in the list of suppliers contacted by investigators.

The investigation came to light yesterday when LG Philips revealed that it had been subpoenaed by regulators in the US, South Korea and Japan. Samsung then revealed that it too had been served with subpoenas, and now Sharp has announced that was summoned by the Japan Fair Trade Commission and the US Department of Justice.

The probe centres on TFT (thin-film transistor) LCDs, according to Samsung, which are used in a wide range of electronics products including flat-panel televisions and computer monitors, laptops, mobile phones and digital music players. The three companies being investigated are among the largest manufacturers of such displays.

Sharp Spokeswoman Miyuki Nakayama said the company was surprised to have received the summons. Sharp has a policy of "fair and ethical management", she said.

Shares in LG Philips dropped 4 percent today, while Samsung's closed down two-thirds of a percent. Sharp waited until after the market had closed to announce it had been contacted. Its shares rose by 2 percent today.