Millions of T-Mobile US customers have had their data stolen as a result of a major breach at Experian.

The consumer credit monitoring firm said that the breach had occurred on 15th September, and concerns customers who applied for T-Mobile USA services between September 1 2013 and September 16 this year.

T-Mobile boss John Legere said he was "extremely angry" about the breach, in a letter to customers posted on the firm’s website.

In his letter, Legere said: "Right now my top concern and first focus is assisting any and all consumers affected. I take our customer and prospective customer privacy very seriously."

Experian said that its consumer credit database was not accessed, nor was any payment card or banking information stolen during the attack.

In a statement the company said: "The unauthorized access was in an isolated incident over a limited period of time. It included access to a server that contained personal information for consumers who applied for T-Mobile USA postpaid services between Sept. 1, 2013 and Sept. 16, 2015."

"Records containing a name, address, Social Security number, date of birth, identification number (typically a driver’s license, military ID, or passport number) and additional information used in T- Mobile’s own credit assessment were accessed. No payment card or banking information was obtained."

While Experian currently believe that the stolen data has not been used by the hackers, it is offering complimentary identity resolution services to the victims. However,