The U.K. is investigating Apple Inc.âs AAPL +0.39% treatment of app developers that offer their wares on the companyâs App Store, adding a new front to antitrust complaints against the iPhone maker. The Competition and Markets Authority, the U.K.âs antitrust regulator, said Thursday that it has opened an investigation into whether Apple imposes unfair or anticompetitive conditions on app developers, including requirements that some types of apps use Appleâs in-app payment system, from which Apple can take commissions of up to 30%. The authority said it opened the investigation following complaints from developers about some of Appleâs terms, such as the requirement that all apps on iPhones and iPads must be distributed through Appleâs store. The authority said it would conduct its initial investigation through September, and no decision has been made about whether the company broke any laws.