Three’s UK network has been hit by a major data breach that could have exposed the personal details of millions of customers.

However, Irish customers of the mobile network have not been affected by the attack on the customer upgrade database, which is believed to have been carried out using compromised employee log-ins.

"Three Ireland is not affected. The system impacted is used solely by Three UK," a spokeswoman for the Irish network said.

The UK network said it was still looking into how many of its nine million customers had been affected. The affected database did not contain customers’ financial details, a spokesman for Three UK said. However, it did contain personal data such a names and addresses.

That information was used to apply for upgraded handsets for a number of customers, which were then intercepted.

Three men have been arrested in connection with the breach.