Apple has officially announced a new subscription service available to all publishers of content-based apps on the App Store.

The system, the same as that used by News Corp's new digital newspaper for iPad The Daily, is available to publishers of apps that offer content that is regularly updated, such as magazines, newspapers and video and music services.

Subscriptions purchased from within the App Store use the same App Store billing system that is currently used for apps and in-app Purchases.

Publishers can set the price and length of subscription and customers can choose their preferred subscription length with one click and are automatically charged.

"Our philosophy is simple—when Apple brings a new subscriber to the app, Apple earns a 30 percent share; when the publisher brings an existing or new subscriber to the app, the publisher keeps 100 percent and Apple earns nothing," said Steve Jobs, Apple’s CEO.

"All we require is that, if a publisher is making a subscription offer outside of the app, the same (or better) offer be made inside the app, so that customers can easily subscribe with one-click right in the app. We believe that this innovative subscription service will provide publishers with a brand new opportunity to expand digital access to their content onto the iPad, iPod touch and iPhone, delighting both new and existing subscribers," he continued.

The way Apple's system works is consistent with what it said in its recent rebuttal of Sony's complaints that Apple had refused to let a Sony Reader app into the App Store.

An Apple spokeswoman said at the time: "We are now requiring that if an app offers customers the ability to purchase books outside of the app, that the same option is also available to customers from within the app with in-app purchase."

This seems to suggest that Amazon will be required to offer an in-app purchasing mechanism based on Apple's API or face removal from the App Store. At the moment, Amazon's Kindle app offers access to content that had been purchased by the app's owner, though to actually buy content, the Kindle app opens the Safari browser on an iOS device where the purchase can be made, bypassing iTunes.