JPMorgan Chase & Co. is suing the former boss of Bruno Iksil, the trader known as the London Whale, for the outsized derivatives positions he took that cost the bank $6.2 billion.

Javier Martin-Artajo, who was Iksil's immediate supervisor, was named as a defendant in a lawsuit filed at London's High Court on Oct. 22.

Details of the court filing will not be released until the defendant acknowledges the claim, the court said.

Greg Campbell, the partner at law firm Mishcon de Reya acting for Martin-Artajo, said his client was confident that when a complete and fair reconstruction of events was completed, he would be cleared of any wrongdoing.

He added that JPMorgan has so far failed to provide details of the evidence to support its allegations, which would enable Martin-Artajo to begin the process of his defense.

"As we have said before, Mr Martin-Artajo is deeply disappointed by the bank's unjustified assertion that he may have attempted to conceal the losses being suffered by the book," Campbell said in an emailed statement.

"There was no direct or indirect attempt by him at any time to conceal losses," the lawyer added.

JPMorgan executives said in July the bank would act to take back pay from traders and supervisors involved in the loss.

A JPMorgan spokeswoman in New York declined to comment.