The mobile provider said it’s working with police and relevant authorities after uncovering the activity which attempted to intercept customer upgrade orders.

It said its upgrade system had been accessed but the system does not include any customer payment, card information or bank account information.

Three Mobile has 9.2 million customers in total but while it has assured them that financial details were not put at risk, it does not yet know how many are affected, though it will be contacting them as soon as possible.

However, there is a chance that personal details such as names and contact details may have been accessed.

In a statement issued today, it said that over the last four weeks it has seen an increasing level of attempted handset fraud, visible through higher levels of burglaries of retail stores and attempts to unlawfully intercept upgrade devices.

Three confirmed that approximately 400 high value handsets had been stolen through burglaries and eight devices have been illegally obtained through the upgrade activity.

The investigation is ongoing but the National Crime Agency said three men had been released on bail pending further enquiries.

A Three Mobile spokesman said it will be releasing further details in due course.