HSBC said that its Swiss arm had not been fully integrated into HSBC after its purchase in 1999, allowing "significantly lower" standards of compliance and due diligence to persist.
The Guardian alleged in its report that the files showed HSBC's Swiss bank routinely allowed clients to withdraw "bricks" of cash, often in foreign currencies which were of little use in Switzerland, marketed schemes which were likely to enable wealthy clients to avoid European taxes and colluded with some to conceal undeclared accounts from domestic tax authorities.
HSBC said the Swiss private banking industry, long known for its secrecy, operated differently in the past and this may have resulted in HSBC having had "a number of clients that may not have been fully compliant with their applicable tax obligations."
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Its private bank, especially its Swiss arm, had undergone "a radical transformation" in recent years, it said in a detailed four-page statement.
HSBC's Swiss private bank was largely acquired as part of its purchase of Republic National Bank of New York and Safra Republic Holdings, a U.S. private bank.
HSBC said the number of accounts in its Swiss private bank had fallen from 30,412 in 2007 to 10,343 at the end of last year and it was cooperating with authorities investigating tax matters.
The data was supplied by Herve Falciani, a former IT employee of HSBC's Swiss private bank. HSBC said Falciani downloaded details of accounts and clients at the end of 2006 and early 2007. French authorities have obtained data on thousands of the customers and shared them with tax authorities elsewhere, including Argentina.
Switzerland has charged Falciani, who Reuters was unable to reach for comment, with industrial espionage and breaching the country's secrecy laws. Falciani has previously told Reuters he is a whistleblower trying to help governments track down citizens who used Swiss accounts to evade tax. Some of the details of the list have been released before. The names of 2,000 Greeks with HSBC accounts was made public in 2010 and dubbed the "Lagarde List" after former French finance minister Christine Lagarde. France passed the names to Greece to help it crack down on tax evasion.