Foreign currency service Travelex said it has taken all its sites offline after being attacked by a software virus on New Year's Eve.
The group said it first discovered the attack on December 31 and immediately took its systems offline to protect data and prevent the virus spreading.
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."