Apple is facing a class-action lawsuit in the UK. The company is accused of imposing a 30% commission on app purchases through its App Store, allegedly abusing its market dominance.

The lawsuit, representing about 20 million iPhone and iPad users, claims that British consumers were overcharged by up to £1.5 billion (US$1.8 billion).

The trial began on Monday at London’s Competition Appeal Tribunal and is expected to last seven weeks.

Apple denies the allegations, justifying the fee due to its ecosystem’s security, privacy benefits and notes that most app developers do not pay the commission.