Microsoft has been fined 280.5m euros ($357m; £194m) by the European Commission for failing to comply with an anti-competition ruling.
The software giant was hit by the fine following a long-running dispute between the US firm and EU regulators.
The move follows a landmark EU ruling in 2004, which ordered Microsoft to provide rivals with information about its Windows operating system.
EU regulators also warned Microsoft it could face new fines of 3m euros a day.
The daily fines will come into force from 31 July if Microsoft fails to supply "complete and accurate" technical information to rival developers, the EU said.
Microsoft has insisted it is meeting the Commission"s demands, and says it expects to deliver the final bundle of information for use by rival software firms by 18 July.