If your iPhone conks out after three years, you might feel shortchanged.

But for Apple it's all part of the plan. On its website, the US technology firm admits it expects the mobile devices to last only 36 months.

Its watches and iPads are also assumed to last around the same time. Owners of Mac computers can hope to eke out a fourth year of use.

The relatively short lifespans come despite some steep prices: the iPhone 6s Plus is £789 and an Apple watch with a solid gold case costs more than £10,000 (NZ$20,512). A 27in iMac is £1,849 (NZ$3,792).

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The revelation comes in a section of Apple's website covering carbon emissions. Kyle Wiens, who founded the tech repair firm iFixit, said: "The majority of the environmental impact of electronic devices is during manufacturing. Apple's claim that we should only use a computer for four years is preposterous and wasteful.

"Also, I'd note that the Apple watch won't last three years without a battery swap, as it lasts 400 charge cycles, or around two years of daily use. Apple doesn't sell batteries to consumers to do that themselves or to independent watch repair shops."

Apple is mounting an environmental push and, over the next ten days, proceeds of 27 popular apps will go to the World Wildlife Fund to mark Earth Day.