Apple hasn't officially confirmed that it is blocking apps that use UDID data or given any reason why it doesn't want apps to access UDID data any more, but it is presumed that privacy concerns lie behind the decision.

Apple had told developers late last year that it planned to block the ability of iOS apps to access the UDID but in order to satisfy lawmakers who have put iOS under scrutiny as part of a wider investigation into how mobile apps impact on individual's privacy this policy appears to have been brought forward.

As Wes Biggs, the co-founder and CTO of mobile advertising firm explains, UDID data is invaluable to developers and mobile advertising firms.

"As the iOS application ecosystem has grown in the past three years, UDID has emerged as the de facto way of enabling a host of functions that make developers' lives easier. By virtue of its widespread adoption and usefulness in tracking how advertising impacts App Store downloads and purchases, it underpins the ad-funded iOS app economy."

However, as Biggs concedes, it does present some privacy problems. "The identifier is permanent and tied to a particular device in a way that is irrevocable and ultimately does not give the user choice over whether to share it with app developers, and by extension, ad networks and analytics companies."