MADRID (Reuters) - Spanish police visited Santander's headquarters <SAN.MC> on Friday as part of a money laundering probe triggered by leaks of sensitive tax information from HSBC'S Swiss private bank.

A High Court spokesman said police, acting on the instruction of the court, had sought documents related to a number of accounts during a visit to Santander's Madrid headquarters, but did not elaborate.

Police confirmed the visit but declined to comment on the nature of the investigation. The probe is linked to tax avoidance but how much money is involved is unclear at this stage, a court source said.

Santander did not return several calls to seek comment. An HSBC spokeswoman in London declined to comment.

The Spanish investigation began after Herve Falciani, a former IT employee at HSBC's Geneva private bank, leaked information on clients and their tax situation. The French daily Le Monde has said it identified more than 106,000 clients.

Falciani has said he is a whistleblower trying to help governments track down citizens who used Swiss accounts to evade tax. France, Austria, Belgium and Argentina have also launched their own investigations.

Swiss authorities say Falciani is a thief who betrayed his employer. He was sentenced to five years in prison for in a Swiss trial last year aggravated industrial espionage.

However, he may not serve time in Switzerland since he lives in France and there are no legal proceedings against him in there. France does not typically extradite its own citizens.

(Reporting By Maria Vega Paul and Sonya Dowsett; Editing by Julien Toyer and Jon Boyle)