Although Apple began rejecting apps that access UDIDs a year ago, the company has now issued a hard deadline for iOS developers who are still creating or updating apps that ignore the changes made in the iOS6 software upgrade.

“Starting May 1, the App Store will no longer accept new apps or app updates that access UDIDs. Please update your apps and servers to associate users with the Vendor or Advertising identifiers introduced in iOS 6.”

Aware of the changes, many third-party ad platforms have either incorporated Apples own identifiers for advertisers into their SDK, like Fiksu, or claim to have created alternative software, like AdTruths device recognition technology. Any Apple users who have not upgraded to iOS6 will effectively be untrackable after this time.

“There have been warnings that Apple would one day ban the use of accessing UDIDs for ad tracking for a while now, especially since they have offered up Apple Identifier (IDFA) as an alternative last autumn,” said Christian Ruck, product and research manager of mobile at Adtech.

“There are both pros and cons to IDFA: we now have a robust mechanism for things like conversion tracking or re-targeting, and it goes some way to partly cleaning up the various user ID technologies that have sprung up,” he said. “But consumers do now have the option to turn it off easily at anytime, and it is still limited to those who have upgraded to iOS 6.0 and beyond.”

From 1 May, new apps must also now be built for iOS devices to support the 4” retina display of the iPhone 5.