Irish-based customers of Booking.com are among those that have been contacted by the reservations website about a potential data breach.
In an email to customers, seen by RTÉ News, the company states that "unauthorised third parties may have been able to access certain booking information associated with your reservation".
Booking.com said it recently noticed suspicious activity affecting a number of reservations and it immediately took action to contain the issue.
"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," the email states.
It goes on to explain how updated PIN numbers have been issued to keep bookings secure.
"If you have received suspicious emails or phone calls, these could be from malicious actors pretending to represent the accommodation or Booking.com," according to the email.
"We'll never ask you to share credit card details by email, over the phone, through text or WhatsApp," the email said.
"We'll never ask you to make a bank transfer that is different from the payment policy details in your booking confirmation," it added.
The email urges customers to remain vigilant before clicking on any links.
Conor Scolard, Director of Cyber Resilience at cybersecurity firm Ekco, said the incident may affect many people in Ireland who have already made their summer travel plans.
"Unfortunately, those who received the security email alert from Booking.com could see an increase in attempted scams in the coming weeks," Mr Scolard said.
"Our advice to anyone who has received the email is to remain vigilant and question any communication about an upcoming or previous booking," he said.
"In particular, anyone who receives apparent direct contact from their hotel should be on high alert," he added.
