1.3 million customers of the French branch of Orange have had their personal data stolen by hackers.
The hackers have acquired names, phone numbers and email addresses.
Orange France delayed announcing the hack in order to assess the damage caused, having been aware of it since 18 April.
Orange experienced a similar security breach earlier this year when 800,000 of its clients had data stolen.
"For the people concerned, the data recovered includes their first names and surnames," an Orange representative said.
"In addition, depending on the information supplied, email addresses, mobile and fixed-line phone numbers, the identity of the person's internet operator and their date of birth, were also recovered."
Unable to say whether the stolen data was encrypted, Orange has warned customers that those targeted could be contacted via phone, SMS or email, for phishing – a process that acquires sensitive data by duping users through social engineering techniques.
The company has been lambasted by customers on social media, notably Facebook and Twitter.
"All necessary actions have been implemented to correct the relevant technical dysfunctions and to prevent any new illegitimate access to this data," said an Orange representative.
A similar case occurred back in September 2013, when a hacker claimed to have stolen the data of two million Vodafone customers in Germany.