increasingly closely with officials at the National Cyber Security Centre on better prevention techniques.

Last December, Megan Butler, the FCA's director of supervision, said the regulator believed that there was "currently a material under-reporting of successful cyberattacks" by British banks.

"The number of breaches relayed back to us looks modest when you set it against the number of attacks on the industry."

Tesco Bank has a total of six million customers across products such as current and savings accounts, credit cards and loans.

It made more than £200m in profit for its parent - currently the UK's largest retailer - during the last financial year, and has set out plans to expand its presence during the coming years.

Benny Higgins, Tesco Bank's high-profile chief executive at the time of the 2016 incident, hassince retired from the role, and been replaced by Gerry Mallon, who joined from Ulster Bank.

At the time of the cyberattack, Andrew Bailey, the FCA chief executive, said it looked "unprecedented in the UK" and required urgent attention, although the sophistication and intensity of such incidents has evolved at a rapid pace in the nearly-two years since then.

Regulators have also become increasingly concerned about the number of IT systems failures which are unrelated to criminal activity.

Last week, millions of UK customers of banks including Barclays and NatWest - owned by the state-backed Royal Bank of Scotland - were temporarily left without access to their online accounts because of technical failures.

Nicky Morgan, the Conservative MP who chairs the Treasury Select Committee, said the problems were "yet another addition to