Despite several delays and date changes, the company complained earlier this month that it had not been left with enough time to respond.

But in the event, the response came in late on Friday night, just two hours before the midnight deadline the European Commission had imposed.

Neither side would comment on what the defence contained.

The EU's Competition Commissioner had accused the company of illegally exploiting its stranglehold on the personal computer market.

On the one hand, the commissioner said, Microsoft was trying to corner the market for servers, the larger back-end computers which store information for other users to access.

On the other, he charged, it was trying to stifle competition for multimedia players such as Real Networks' Realplayer and Apple's Quicktime.

Huge fines

The accusations - both denied by Microsoft - could, if found to be proven, trigger fines of up to $3bn and force the company to open more of the code underlying its software to its competitors.

The company was found to have a monopoly which it abused by a US court in 2000, although the harsh remedies the judge had proposed were substantially watered down on appeal, with the acquiescence of the Bush administration's Department of Justice.

But that case revolved around internet browsers, and tackled the past.