Tesco has enlisted the help of spy agency GCHQ to assist its investigation of what is the most serious cyber attack ever launched against a British bank.

The supermarket giant contacted the National Cyber Security Centre (NCSC), a new part of GCHQ that tackles crime online and opened last month, after it learned of the theft at Tesco Bank at the weekend.

The NCSC reports into GCHQ, the UK’s digital espionage agency, and has been providing “on-site assistance” to Tesco. It is working alongside the National Crime Agency to investigate the attack.