Computer giant Microsoft has told a European judge that it should not have to share its technological secrets with its rivals.
Microsoft's operating system is used on nine out of 10 home computers
In March, the European Commission ordered the firm to share details of its software systems after it was said to have abused its monopoly position.
Microsoft is in court to have the ruling temporarily suspended before it tries to permanently overturn it.
The software giant argues that the ruling will do it irreparable damage.
The March ruling included a 497m euro ($613m; £340m) fine.
Microsoft's attempt to have the ruling suspended is being heard before the Court of First Instance in Luxembourg on Thursday and Friday.
"This is the first time in competitive history that a company has been ordered to draw up a description of secret technology and deliver it to its competitors," Microsoft lawyer Ian Forrester said before the Court of First Instance in Luxembourg.
Yet speaking in court against Microsoft, one of its software competitors - Samba - said that in the past Microsoft had freely shared details of its core software systems or protocols.
"The protocols are not secret because they're valuable;
they're valuable because they're secret," said Samba co-creator
He said that each time Microsoft updates its products, it
can tweak the protocols so others cannot use them.
"Microsoft builds on the work of others but makes small but
critical changes," said Mr Allison, comparing it to a telephone company
that will not tell competitors how to dial phone numbers.
The Court of First Instance's decision is expected to be delivered in a matter of weeks.
Whatever its verdict Microsoft will continue with its legal appeal to have the ruling permanently overturned, a case which is expected to last up to three years.
In its March ruling, the European Commission also said Microsoft must provide a pared-down version of its Windows operating system, minus its own media player, so that rival systems such as RealNetworks have a greater chance of being used.
This matter will be heard before the Court of First Instance on Friday.