Apple has responded to claims of a discrepancy in how the iPhone's processor should work as its battery ages compared with how it responds in the real world.

The California-headquartered company admitted on Thursday that a software update, released last year, does make the phone work more slowly.

Apple acknowledged that the update reduces power demands when an older battery struggles to supply the peak power that the processor demands. That measure has the effect of slowing the phone's operations, it said.

"Last year we released a feature...to smooth out the instantaneous peaks only when needed to prevent the device from unexpectedly shutting down," Apple said in an emailed statement to the Reuters news agency.

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The iPhone maker said the feature has been extended to the iPhone 7 and will be used on other products in the future.

Independent tests conducted

Earlier this week, Primate Labs, a firm which produces an app that measures the iPhone's processor speed, said it noticed a slow down and lower performance in older models of the smartphone.

The company's founder John Poole said in a blog post that Apple's iOS operating system likely slows down the phones' performance when the battery deteriorates to a certain level.

"The problem is due, in part, to a change in iOS. The difference between [the two versions] 10.2.0 and 10.2.1 is too abrupt to be just a function of battery condition," he wrote.

"I believe (as do others) that Apple introduced a change to limit performance when battery condition decreases past a certain point."

Poole said the issue was "widespread" and predicted that more users would be affected in the future.

Theories proved partially true?

Claims that iPhones slow down over time have been reported on regularly since the smartphones first appeared over a decade ago.

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Users have long complained that phone manufacturers like Apple utilize so-called planned obsolescence techniques where devices wear out much faster than expected, forcing users to buy the newest model.

But Apple insisted the measure was only introduced after it noticed a problem was becoming widespread where iPhones would suddenly shut down as the software attempted to protect the processor from being damaged by sudden power spikes caused partly by aging batteries.

It said lithium-ion batteries used by all phone manufacturers can face problems as they get old, are cold or low on charge.

Several tech websites said rather than splash out on new phones, users can get Apple to replace the battery for a $79 fee in the US or €89 euros in Europe.