The bank identified "suspicious activity" via its fraud prevention systems on Saturday and yesterday warned thousands of customers that they may have been targeted by fraudsters.
Today, chief executive Benny Higgins said online transactions from current accounts had been halted temporarily as a precautionary measure.
"This will only affect current account customers," he said.
"While online transactions will not be available, current account customers will still be able to use their cards for cash withdrawals, chip and pin payments, and all existing bill payments and direct debits will continue as normal. We are working hard to resume normal service on current accounts as soon as possible."
We apologise for the worry and inconvenience that this has caused for customers, and can only stress that we are taking every step to protect our customers’ accounts.
Higgins added that the bank is working with the authorities and regulators to address the fraud and will keep customers informed through regular updates on its website, on Twitter and with direct communication.
He also said any financial loss incurred by customers as a result of the fraud will be "resolved fully by Tesco Bank".