A former JPMorgan Chase & Co. trader wanted by the United States for allegedly falsifying bank records to cover up $6 billion in trading losses has told Spain's National Court he will challenge extradition.

A court spokesman said Friday that Javier Martin-Artajo, 49, opposes the petition because he is Spanish. The court will now study the U.S. request and hold a hearing at a later date. The official spoke on condition of anonymity in keeping with court regulations.

The U.S. accuses Martin-Artajo and another ex-trader at the bank of marking up the value of an investment portfolio to hide its plummeting value. The portfolio eventually incurred a $6 billion loss.

Martin-Artajo was arrested in Madrid in August but freed without bail. He denies the U.S. charges.