Almost a quarter of a million of Wonga's UK customers may have been affected by a data breach.
The payday lender said it is investigating "illegal and unauthorised access" to some of its customers' personal information in both Britain and Poland.
Stolen data may include account numbers, sort codes, addresses and the last four digits of users' bank cards.
Around 245,000 customers may have been affected in the UK, and 25,000 in Poland, a spokeswoman confirmed.
The firm, which said it knew something had happened by Tuesday but did not become aware of a data breach until Friday, said it believes users' accounts are secure and no action needs to be taken.
Customers have been warned to look out for any "unusual activity".
The spokeswoman added: "Wonga is urgently investigating illegal and unauthorised access to the personal data of some of its customers in the UK and Poland.
"We are working closely with authorities and we are in the process of informing affected customers.
"We sincerely apologise for the inconvenience caused."
The police and Information Commissioner's Office have been informed.