Travel money firm Travelex has taken all its websites down after being attacked by a software virus on New Year's Eve.
The group said it immediately took down its online systems to protect data and prevent the virus spreading after discovering the attack on Tuesday.
It has since been forced to provide foreign exchange services manually in its branches.
Travelex stressed its investigations so far suggest no personal or customer data has been compromised.
Tony D'Souza, chief executive of Travelex, said: "We regret having to suspend some of our services in order to contain the virus and protect data.
"We apologise to all our customers for any inconvenience caused as a result.
"We are doing all we can to restore our full services as soon as possible."
Travelex said it has drafted in teams of IT specialists and external cyber security experts to isolate the virus and get affected systems back online.
It comes as many holidaymakers are abroad for the Christmas holidays.
Travelex was at the centre of an IT crisis nearly two years ago, when it mistakenly leaked data on thousands of customers of partner Tesco Bank.
The group, which provided foreign currency on behalf of Tesco Bank, shared the customer database by mistake in March 2018, exposing the details of 17,000 people, including full names, emails, phone numbers, IP addresses and the final digits of bank cards.
