The practice emerged in April, when two British security researchers found the fil on both iPhone handsets and the computers with which they were associated. The unencrypted location data it contained stretched back almost a year.

In May, Apple was ordered by a South Korean court to pay Kim Hyung-souk, a lawyer, one million won (£565) in compensation as a result.

His firm, Mirae law, is now representing 27,000 iPhone users in a class-action. Each is also seeking the same compensation, which would mean a total payout of more than £15.2m if their claim is successful.

“I’m an iPhone user myself, so when I first heard about this in the media, I reviewed the legality of the matter based on Korean law,” Mr Kim said.

“I concluded it was clearly illegal.”

Apple has declined to comment on the case.

A week after the existence of the hidden log was publicised, Apple announced that it would update iOS to fix a “bug” that made storing location data hard to turn off. It also said it would stop backing up the file on the computer associated with each iPhone, and encrypt it.

Apple said it had been “maintaining a database of WiFi hotspots and cell towers around your current location, some of which may be located more than one hundred miles away from your iPhone, to help your iPhone rapidly and accurately calculate its location when requested”.
The update has since been released.