Citigroup has finally admitted that a cyber attack in May affected over 360,000 of its customers. This is twice the initial number that the bank's figures had suggested and follows pressure to release more details about the hack.

Citigroup is the third largest US bank. The breach, which Citigroup first made public on June 8, was initially detected on May 10.

More than 360,080 North American Citigroup credit card accounts were affected by the breach, the bank said. This is only one percent of North American card customers had been affected.

But the bank's annual report says that number of customers totals 21 million, which initially indicated that about 200,000, rather than over 360,000, customers had been affected. Citigroup reissued credit cards to 217,657 accounts along with a notification letter.