Microsoft has backed calls for an investigation into app store monopolies, piling more pressure on Apple as more and more developers hit out at the company.
Speaking at an event hosted by Politico, Microsoft president Brad Smith said that the nature of app stores needed to be "justified".
Although he didn't name Apple specifically, the Cupertino company is currently at the centre of an App Store backlash, following mass reports of its heavy taxes on developers and its strict rules for entering its app ecosystem.
Last week the European Commission announced it was launching an antitrust investigation into its App Store rules and the Apple Pay platform. The case will look at the way Apple forces developers to use its proprietary in-app purchase system and how it restricts their ability to inform iPhone users of alternative ways to purchase the apps.
A number of app developers have voiced their frustrations over Apple's policies and an initial complaint from Spotify has led to a formal investigation. Over the weekend, Microsoft added to the discourse.
"They impose requirements that increasingly say there is only one way to get on to our platform and that is to go through the gate that we ourselves have created," Smith said. "In some cases, they create a very high price per toll - in some cases, 30% of your revenue has to go to the toll keeper."
"The time has come - whether we are talking about DC or Brussels - for a much more focused conversation about the nature of app stores, the rules that are being put in place, the prices and the tolls that are being extracted and whether there is really a justification in antitrust law for everything that has been created."
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There is currently an ongoing conversation between Apple and developer Basecamp about the app of a premium email service called 'Hey'. The iPhone maker sent a letter to Basecamp, according to The Verge, reiterating its App Store rules which it accuses Hey of breaking.
Basecamp has previously hit out at Apple, saying it shouldn't be forced to hand over 30% of its annual $99 subscription fee via in-app purchases. The email service is launching next month but is currently available as a free trial for invited users.
