Spain's National Court is holding an extradition hearing for a former bank employee wanted by Switzerland for allegedly stealing confidential information related to thousands of customers with Swiss accounts.

Herve Falciani, a French-Italian citizen, allegedly removed data linked to at least 24,000 customers of HSBC's Swiss subsidiary, potentially exposing many to prosecution by tax authorities in their home countries. He passed a list of names to French authorities.

France's former Finance Minister, Christine Lagarde, now head of the International Monetary Fund, relayed the list to the U.S. and many European Union countries.

Spanish politicians oppose Falciani's extradition, saying he is helping authorities investigate tax fraud, a touchy issue given Spain's recession and sky-high unemployment.

French prosecutor Eric de Montgolfier is due to attend Monday's hearing as a witness.