Sun Microsystems, a rival of Microsoft, yesterday upstaged the US Justice Department and several state attorneys-general by filing a legal motion seeking to block shipments of Windows 98 unless it is modified.

In a federal court filing, Sun - a Silicon Valley computer manufacturer and software developer - said that Microsoft's new personal computer operating system would become a "massive distribution vehicle" for disputed technology.

Federal and state anti-trust regulators are expected to file new charges against Microsoft within days, which may also include demands that the software company delays distribution of Windows 98. The government has been investigating whether Microsoft has used exclusionary or predatory practices to restrain competition.

Microsoft dismissed the legal motion as a "publicity stunt". "The timing is suspicious," said Mr Tod Nielsen, manager of Microsoft's developer relations. Sun was seeking to ride the wave of public interest in Windows 98, he added.

Windows 98, which is scheduled to be shipped to PC manufacturers on Friday and introduced as a retail product next month, is an updated version of the most widely used PC operating system.

Mr Bill Gates, Microsoft chairman and chief executive, said last week that any government action to disrupt shipment of Windows 98 would "hurt the American economy and would cost American jobs". The effects, he said, "would be profound and would ripple through the economy".

Yesterday's action by Sun is separate to any legal moves by the government or states, and concerns the Sun-developed Java, the new lingua franca of the computer world.

Sun argues that Microsoft has `deceptively modified' its Java software which Microsoft has under licence and used in Windows 98.