GETTY The hackers used an employee's login to gain access to the millions of customers

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The company said hackers used an employee login to access its customer upgrade database, leaving nine million customers at risk. A spokesman for Three said: "Over the last four weeks, Three has seen an increasing level of attempted handset fraud. This has been visible through higher levels of burglaries of retail stores and attempts to unlawfully intercept upgrade devices.

The investigation is ongoing and we have taken a number of steps to further strengthen our controls A spokesman for Three

"We’ve been working closely with the Police and relevant authorities. To date, we have confirmed approximately 400 high value handsets have been stolen through burglaries and eight devices have been illegally obtained through the upgrade activity.

"The investigation is ongoing and we have taken a number of steps to further strengthen our controls. "In order to commit this type of upgrade handset fraud, the perpetrators used authorised logins to Three’s upgrade system.

GETTY The breach occurred on Thursday evening

"This upgrade system does not include any customer payment, card information or bank account information." Three only discovered the breach, which occurred on Thursday evening, after customers filed complaints claiming that scam callers were attempting to gain access to their bank accounts.

GETTY Customer's whose information have been breached, are yet to be informed

The hackers accessed customer accounts and upgraded them in order to intercept the new phones and sell them on. Earlier this month, Phillip Hammond said companies have a duty to protect their customers against cyber crime following a series of high-profile breaches.

The Chancellor said: "Trust in the internet and the infrastructure on which it relies is fundamental to our economic future. Because without that trust in, faith in the whole digital edifice will fall away." The National Crime Agency is investigating the breach and said that three people have been arrested, two for computer misuse and one for perverting the course of justice.

GETTY Philip Hammond urged companies to do their best to protect customers from cyber attack