The European Commission yesterday extended its antitrust investigations of Microsoft to cover allegations that it abused its near-monopoly in personal computer operating software to drive out competitors in the fast-growing market for server software.

The move is another blow for Microsoft, which could be forced to break itself up after being found guilty of violating US antitrust laws earlier this year.

In February the commission launched a similar probe of Microsoft for alleged abuse of dominance linked to its latest windows 2000 software in the server market.

If the commission finds against the Seattle-based company, it could impose fines of up to 10 per cent of its global revenues.

Mr Mario Monti, EU competition commissioner, said he had sent a "statement of objections" to Microsoft following a complaint from Sun Microsystems, the US server software producer, that Microsoft breached EU antitrust rules through discriminatory licensing and refusing to supply essential information about its Windows operating system.

Sun claimed Microsoft, because of its 95 per cent share of the market for personal computer operating systems, was obliged to disclose the links or interfaces that would enable its systems to operate with non-Microsoft software for servers.

Servers form the basis for network computing and, according to the commission, "constitute a strategic sector in the development of a global market for information technology and e-commerce".

Microsoft argued yesterday that Sun's complaint was "based on its desire to gain access to technical trade secrets".