Fitness brand Under Armour said personal details of around 150 million users of its MyFitnessPal nutrition app and website were accessed in a breach earlier this year.
The US company said it discovered the breach last Sunday, with data stolen including passwords, usernames and email addresses.
The breach had occurred in late February, and Under Amour has been taking steps to alert users about the incident, it said.
Payment details, driving licences and social security numbers were not accessed by the unauthorised party, as these details are "collected and processed separately", it added.
The MyFitnessPal app and website, which allows people to track their calorie intake and exercise routines,...