Shares in Dixons Carphone, the company behind Carphone Warehouse, which over the weekend admitted it had been targeted in a "sophisticated" cyber attack which left 2.4m customers' payment details vulnerable, fell 1.7 per cent to 448.26p as the market opened.
However, they later regained some of those losses, rising to 450p, 1.3 per cent lower than Friday's closing price.
Data watchdog the Information Commissioners Office said on Sunday it was making "enquiries" into the company, after it warned encrypted credit card details of up to 90,000 customers may have been part of the data taken by hackers during an attack on 5 August.
Chief executive Sebastian James said the company took the security of customer data "extremely seriously".
"We are, of course, informing anyone that may have been affected, and have put in place additional security measures," he added.