Apple has been charged with breaking EU competition rules over the way it runs its App Store.
European Commission anti-trust regulator Margrethe Vestager tweeted that "consumers are losing out".
It relates to charges brought two years ago by music streaming app Spotify which claimed that Apple was stifling innovation in that industry.
Apple faces a large fine and may be forced to change its policies if its arguments do not convince regulators.
Previously it has denied any wrong-doing.
The case is looking specifically at how its App Store policies affect music streaming. The charge was initially filed in 2019 by co-founder of Spotify Daniel Ek, who said that Apple was "limiting choice and stifling innovation".