HSBC was on Thursday rushing to contact all the wealthy clients of its Swiss private bank after it admitted that data affecting 24,000 current and former clients had been stolen. In December, the bank said it believed fewer than 10 accounts were affected when Herve Falciani, a former HSBC IT specialist, stole the data and passed it to the French tax authorities. HSBC admitted on March 11 that it had now discovered that 15,000 existing and 9,000 former clients were affected. The bank is contacting those clients and any others with Swiss private bank accounts to explain and apologize. The private bank has 100,000 clients globally. HSBC said the earlier estimate was made with the information it had from the Swiss authorities and that it only had the full picture on March 3, when the Swiss returned “a significant portion” of further data. The bank stressed that the data was the subject of a criminal investigation, making it difficult to assess the scale of the damage in December.