The organisation carried out a number of tests to determine whether the average talk time for popular smartphone models lined up with the battery life claims of their manufacturers.
Nine iPhone models were tested, and the investigation found that all fell short of Apple's battery life claims, with Apple stating that its iPhone batteries lasted between 18% and 51% longer than they actually did.
The biggest battery life overstatement was for the iPhone Xr, with the device only lasting for 16 hours and 32 minutes compared to Apple's claim that it would last 25 hours.
HTC also reportedly overstated its battery life claims by around 5%, although the test found that Nokia, Samsung, and Sony all understated their average talk time for a single charge.
When conducting its test, Which? charged up new, independently-purchased smartphones and tested how long lasted when making continuous calls.
The company said it also put smartphones through a number of different tests before making a ruling on battery life.
