Booking.com says some of its customer data has been accessed by hackers.
The accommodation reservation site warned customers of potential suspicious emails and phone calls from “malicious actors”.
However it claimed that “financial information was not accessed”.
An email to customers read: “Unauthorised third parties may have been able to access certain booking information associated with your reservation.
“Based on the findings of our investigation to date, accessed information could include booking details and name(s), emails, addresses, phone numbers associated with the booking and anything that you may have shared with the accommodation.”
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It added: “We’ll never ask you to share credit card details by email, over the phone, through text or WhatsApp.
“We’ll never ask you to make a bank transfer that is different from the payment policy details in your booking confirmation.”
Headquartered in Amsterdam, the firm lists more than 30m accommodation venues around the world.