The commission ruled earlier this year that BAA must sell Gatwick, Stansted and Edinburgh or Glasgow airports within two years.
BAA, which is selling Gatwick anyway, has appealed to the Competition Appeal Tribunal in a case expected to last three days.
BAA will argue there was an “apparent bias” because of a link between a member of the ruling panel and an organisation interested in buying the airports.
On the issue of “proportionality”, BAA says the Commission failed to take into account, or carry out an assessment of, material considerations of selling the airports, particularly in the context of the current financial and economic environment.
In its report, the Commission concluded that the common ownership of south-east England airports by BAA - which also runs Heathrow - adversely affected competition.
At the start of this year BAA said its annual profits had fallen by 18.4% as the economic downturn dented passenger numbers.