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Apple has delayed a major upgrade to its smartwatch's operating system, after finding a bug in the software.

The update had been set to be released alongside the iOS 9 update for its smartphones and tablets.

The Apple Watch is the bestselling smartwatch to date, according to research company IDC.

But the current version of its software means that many third-party apps take a relatively long time to load and cannot use the device's sensors.

WatchOS 2 addresses this by allowing the apps to run natively on the Watch rather than making them rely on a linked smartphone's processor.

"We have discovered a bug in development of WatchOS 2 that is taking a bit longer to fix than we expected," an Apple representative told the TechCrunch news site.

"We will not release WatchOS 2 today but will shortly."

Last September, Apple issued an apology after an update to its iOS mobile system caused some of its phones to become unable to connect to cellular networks.

But it is far from being alone in being affected by upgrade bugs.

On Wednesday, Google rushed out a fix for its Android OS after it was discovered the lock screens of devices running the code could be overridden by typing extremely long text into the password box.

"Apple's news shows that there's a lot of work that's done on these releases down to the last minute," said Carolina Milanesi, chief of research at Kantar Worldpanel ComTech.

"This was the right thing to do. Obviously there's excitement about the update, but there's nothing that the consumer can't wait for for a couple of days or however long it takes."

iOS 9 has been released as originally scheduled.