The origin of the leak

The HSBC files were obtained through an international collaboration of news outlets, including the Guardian, Le Monde, BBC Panorama and the International Consortium of Investigative Journalists
In late 2007, Hervé Falciani, an IT expert at HSBC's Swiss bank, hacked into its customer files. He fled to France with police on his trail for breaching Switzerland's rigid bank secrecy laws.
The French authorities detained him, but refused to extradite him when they realised the data could identify thousands of French tax evaders. Falciani now lives in France under protection.
In early 2010, under finance minister Christine Lagarde, France prepared confidential lists of the leaked names for other countries. The so-called "Lagarde list" led to scandal and arrests in Greece, Spain, the US, Belgium and Argentina.
Britain's tax authority, HMRC, received a list in 2010 from which it identified more than 1,000 tax evaders. More than £135m ($206m) was quietly recovered in repayments, but only one person was prosecuted. There has been no UK legal action against HSBC. Names were never revealed.