Around 79,000 customers are now believed to have been affected by the theft of data from HSBC's Swiss private banking arm by an IT employee, according to a French prosecutor who has launched a tax evasion investigation based on the information.
In December HSBC admitted IT employee Herve Falciani had stolen client data but claimed the theft affected less than 10 people. However, last month it admitted 15,000 client accounts were affected, with details passed on to the French tax authorities.
Now, French prosecutor Eric de Montgolfier says the stolen files have been decrypted, revealing 127,000 accounts belonging to 79,000 people. According to Reuters, de Montgolfier told reporters that of these, 8231 related to French customers with HSBC accounts in Switzerland.
Italian authorities are also reportedly asking for information relating to around 7000 of its nationals.
HSBC says the figures provided by de Montgolfier are "massively overstated".
Falciani stole the data three years ago and fled to France while under investigation. Last January, acting on a Swiss warrant, the French authorities seized the data from his home and are now using it in a money laundering probe.