Tesco Bank has paid out an estimated £2.5 million to 9,000 customers after suffering a hacking attack.
The business said a full service for customers had resumed after the attack at the weekend saw the bank freeze online transactions for customers in an emergency security measure.
All accounts which had money fraudulently removed have now had it replaced, Tesco Bank chief executive Benny Higgins said, adding: "We'd also like to reassure our customers that none of their personal data has been compromised."
The chief executive of the Financial Conduct Authority (FCA) described the hack as "unprecedented" and said authorities were working to find the "root cause" of the breach.
Andrew Bailey told MPs at a Treasury Select Committee hearing that although all money would be refunded to affected customers, "that's important but not by any means the heart of the concern.
"The heart of concern is what is the root cause of this and what it tells us about the broader threats."
The threat appears to have only affected the "debit card side of online banking", but Mr Bailey said "further urgent analysis" is required.
Tesco Bank said that suspicious activity was tracked across 40,000 of its 136,000 current accounts over the weekend.
Customers affected by the freeze on online accounts were still able to withdraw cash and use services such as chip and pin payments, while bill payments and direct debits continued as normal, the bank said.