The Spanish former JPMorgan banker arrested here Tuesday on U.S. charges arising from the bank's $6 billion "London Whale" scandal was released on his own recognizance after vowing to fight extradition to the United States.

Javier Martin-Artajo Rueda, who was head of credit and equity trading at JPM's chief investment office in London, surrendered to Spanish police after authorities trying to execute an international arrest warrant tracked him to his Madrid home.

Fired by JPM in August 2012, Martin-Artajo continued to live in suburban London, but happened to be abroad on vacation when the United States issued a warrant for his arrest, according to U.S. media accounts.

Police brought the former banker before National Court Judge Santiago Pedraz, who released Martin-Artajo on the condition that he not leave Spain and report to court twice a month.

U.S. authorities have 40 days to file a request for extradition.

Prosecutors in the United States accuse Martin-Artajo, 49, and erstwhile colleague Julien Grout of having concealed more than $500 million in losses generated by the JPM London office's derivatives strategy.

JPMorgan Chase & Co. ultimately lost more than $6 billion on derivatives.

The charges against Martin-Artajo and Grout are based on testimony from the trader at the center of the scandal, Bruno Iksil, dubbed the "London Whale" for the size and audacity of his trades.

U.S. prosecutors agreed to give Iksil immunity in exchange for his cooperation.

Besides denouncing the accusations against him as "false," Martin-Artajo showed Judge Pedraz a photocopy of Iksil's statement to U.S. authorities, judicial sources said.

The charges Martin-Artajo faces in the United States carry a possible maximum sentence of 65 years in prison. EFE