The banking arm of supermarket-chain Tesco has now resumed normal service after shutting down the service for several days to protect itself from further losses.

The bank has revealed the theft cost it £2.5 million, taken from around  9,000 customer accounts over the weekend.

The theft saw suspension of online transactions on Monday by the bank for security purposes. Tesco Bank announced yesterday that  it has  now resumed full service. The bank asserted that no customer data had been compromised.

Tesco Bank's CEO, Benny Higgins, said: “We've now refunded all customer accounts affected by fraud and lifted the suspension of online debit transactions so that customers can use their accounts as normal."

The National Cyber Security Centre confirmed on Tuesday that it was working with Tesco and the National Crime Agency to investigate the attack, which has been described as "unprecedented" by the Financial Conduct Authority chief executive, Andrew Bailey.

The bank has still provided very little detail about what exactly happened over the weekend. It is not yet clear how criminal carried out the attack, and there has been no word yet if there are any suspects.

Meanwhile, some security experts have suggested that the cyber-attack on Tesco Bank could be an inside job. The reason this is being predicted is because cyber-criminals managed to steal money from a large number of accounts all in a short space of time.

Jonathan Sander, VP of product strategy at Lieberman Software said: “The shape and scale of the attack on Tesco's customers does seem to suggest that there was an element of someone – or something – on the inside.”