Travelex has taken all of its sites offline after being targeted by a software virus, the foreign currency service has said.
The group said it first discovered the attack on New Year's Day and immediately took its systems down, with its early investigations suggesting that no personal or customer data has been compromised.
Exchange services are having to be handled manually in branches during the outage, the firm added.
Chief executive Tony D'Souza 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."
Cyber security and IT experts are said to be working to isolate the virus and get affected systems back online, but the timing is less than ideal with many holidaymakers still abroad for the Christmas break.
Travelex has a presence in more than 70 countries and has more than 1,200 branches and 1,000 ATMs worldwide, with exchange machines a common sight at airports, and customers can also use a smartphone app.
The London-based firm processes more than 5,000 currency transactions every hour.
The issue comes almost two years after the company was embroiled in another IT crisis, when it mistakenly leaked customer data from thousands of Tesco Bank accounts.
The group, which provided foreign currency on behalf of Tesco Bank, shared the database by mistake in March 2018, exposing the details of 17,000 people.
Those included full names, emails, phone numbers, IP addresses and the final digits of bank cards.