Spanish police has arrested the former JPMorgan manager Javier Martin-Artajo in Madrid, who was charged by US prosecutors with attempting to conceal losses and falsify records relating to the London Whale scandal.

Court officials said that Javier Martin-Artajo declared in court that he would fight extradition to face charges of fraud and falsifying bank records to hide the extent of losses in early 2012 that eventually totaled about US$6 billion.

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Martin-Artajo handed himself in to police after they found him and got in touch with him, Spanish police said in a statement. He was to be brought before the National Court in Madrid as part of extradition proceedings.

"The arrested person is presumed responsible for manipulating and inflating the value of positions in the synthetic credit portfolio of his firm with the aim of achieving specific objectives of daily losses and gains," police said in a statement.

American prosecutors have filed criminal charges against Mr Martin-Artajo and fellow ex-JPMorgan trader, Julien Grout. The pair are alleged to have marked up the market value of an investment portfolio to hide its plummeting value.

The US Department of Justice claims the two were responsible for hiding more than US$660 million (£425 million) in losses.

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The trader who placed the bets, Bruno Iksil, known as the London whale, was not charged, as he is cooperating with authorities.

Martin-Artajo denied the US charges and said he will challenge any extradition request.

The United States has 40 days to request Spain hand over Martin-Artajo to US authorities. The Spanish court ordered the former banker to stay in the country and to make an appearance in court twice a month.