iOS 9 might now be officially available – but its roll-out has been anything but smooth, Digital Spy reports.

Some iPhone, iPad and iPod touch users and developers are reporting difficulty downloading Apple's latest operating system update, with 'Software Update Failed' error messages prevalent.

Fan site 9to5Mac reports the error for over-the-air users could be related to server load trouble.

Earlier on September 16, Apple delayed the release of its iPhone and iPad-friendly iOS 9 patch watchOS at the very last minute.

"We have discovered a bug in development of watchOS 2 that is taking a bit longer to fix than we expected. We will not release watchOS 2 today but will shortly," an official Apple spokesperson told Digital Spy.

iOS 9 was officially announced at Apple's Worldwide Developers Conference in June to much fanfare.