Travelex has been forced to take down its website after a cyber attack.
The foreign-currency seller has been working on the issue since the software virus attack on New Year's Eve.
"We regret having to suspend some of our services in order to contain the virus and protect data," Travelex boss Tony D'Souza said.
The company has resorted to carrying out transactions manually, providing foreign-exchange services over the counter in its branches.
"We apologise to all our customers for any inconvenience caused as a result," Mr D'Souza said in a statement.
"We are doing all we can to restore our full services as soon as possible."
Teams of specialists
The company said an early investigation "shows no indication that any personal or customer data has been compromised".
Travelex said it had deployed "teams of IT specialists and external cyber-security experts", who have been "working continuously since New Year's Eve to isolate the virus and restore affected systems".
The firm will continue to provide foreign-exchange services manually at its branches until the problem is fixed.
The decision to take the site down has affected other services that use Travelex, including Tesco Bank.
Responding to customers on Twitter, the bank said its travel money service was unavailable.
Unfortunately our on-line Travel Money service is currently unavailable due to IT issues at our partner, Travelex. In the meantime, you can still visit one of our in-store bureaux to collect or purchase your currency. Sorry for any inconvenience - Brogan— Tesco Bank Help (@tescobankhelp) January 2, 2020