Marriott International said hackers have stolen about 500 million records from its Starwood Hotels reservation system, reported Reuters.
The attack began four years ago, added the report, and has exposed the personal data of customers including bank card numbers.
The hack started before Marriott began its acquisition of Starwood Hotels in November 2015. This saw it acquire brands Sheraton, Ritz Carlton, and the Autograph Collection.
For 327 million guests whose data has been taken, passport details, phone numbers, and email addresses were exposed.
Marriott said it was made aware of the breach on 8 September, and that it would inform affected guests.