Smartphone giants Apple have apologized for slowing down older iPhone models without telling users first.

Apple admitted to slowing down the iPhone 6, 6S, 7 and SE – when their batteries are either old, cold or have a low charge – to prevent abrupt shutdowns.

The US tech company also announced a $50 (£37) reduction in the cost of iPhone battery replacements, down from $79 to $29, and an iOS (operating system) software update providing updates on iPhone battery health in early 2018.

Apple said the problem was that ageing lithium batteries delivered power unevenly, which could cause iPhones to shut down unexpectedly – endangering the delicate circuits inside.

The company said it intentionally slowed the performance of the older iPhones because, when their batteries wear to a certain level, they can no longer sustain the required current demanded by the phones’ processors.

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When the processor demands more current than the battery can supply, the phone abruptly shuts down to protect its internal components, as was the case with the iPhone 6S – for which Apple was forced to replace batteries.

At least eight separate class-action lawsuits have been filed in the US in relation to the admission. Plaintiffs in California, Illinois and New York all argue that Apple did not have consent to slow their devices.

A statement on Apple’s website said: “We’ve been hearing feedback from our customers about the way we handle performance for iPhones with older batteries and how we have communicated that process. We know that some of you feel Apple has let you down.

“We apologize. There’s been a lot of misunderstanding about this issue, so we would like to clarify and let you know about some changes we’re making.

“First and foremost, we have never – and would never – do anything to intentionally shorten the life of any Apple product, or degrade the user experience to drive customer upgrades.

“Our goal has always been to create products that our customers love, and making iPhones last as long as possible is an important part of that.”