Microsoft is using "abusive behaviour" to dominate the software market, according to the European Union's top antitrust official. 

European Competition Commissioner Neelie Kroes told MEPs that Microsoft is using "illegal tactics" to edge out competition, and threatened the software giant with punitive fines for defying Commission rulings.

"Microsoft is constantly gaining market share, and that is worrying me," she said.

Kroes added that, left to its own devices, Microsoft would engulf the workgroup server market and obliterate any competition.

Microsoft's market share has grown from 35 to 40 per cent in 1999 to around 75 per cent today.

The company's tactics were first investigated by the Commission three years ago, when it was fined a record €497m (£337m) for abusing Windows' dominance of office PC networks.

Commissioners found that Windows PCs are easier to connect using Microsoft products than with products from rivals such as Sun Microsystems and Linux vendors. 

Microsoft appealed against the Commission's demand that it share interoperability information with competitors, claiming that the information is patent-protected intellectual property.

The appeal failed, and last July the Commission imposed a further €280.5m (£190m) fine for failure to comply. Earlier this month the Commission threatened even more fines.

Microsoft has until July to reply to the latest charges.