Shares in Tesco (TSCDY) , Britain's biggest supermarket chain, fell sharply Monday following a weekend cyber attack on 40,000 accounts at its online banking arm.

"Tesco Bank can confirm that, over the weekend, some of its customers' current accounts have been subject to online criminal activity, in some cases resulting in money being withdrawn fraudulently," Tesco Bank CEO Benny Higgins said in a statement. 

Wholly-owned Tesco Bank, which has 136,000 current accounts, has frozen all online banking transactions from current accounts and said it would refund those which had money stolen. Customers will be allowed to use cards to withdraw cash and to make payments, Higgins said.

"We can reassure customers that any financial loss as a result of this activity will be resolved fully by Tesco Bank, and we are working to refund accounts that have been subject to fraud as soon as possible," he added.

The bank didn't disclose how much money was stolen from its customers.

Tesco Bank accounted for almost 15% of group operating profit in the first half of the fiscal year and as of April, it had 7.6 million customers for all its financial products.

Tesco shares lost 3% in early London trading but pared those losses to around 1.1% by mid-day, with shares in the group changing hands at 200.30 pence each.

The bank has been growing quickly as a competitor to Barclays(BCS) - Get Report , Lloyds(LYG) - Get Report and Royal Bank of Scotland(RBS) - Get Report by offering 3% interest on balances of up to £3,000 ($3,724.68). 

The U.K.'s Financial Conduct Authority has said it is monitoring the hack.