The US case against two former JPMorgan traders charged with concealing billions of dollars in losses fell apart because a key witness known as the London Whale shifted blame to chief executive Jamie Dimon and other top executives, according to a person familiar with the matter.

Four years ago, Bruno Iksil, a former JPMorgan trader in London, agreed to testify against ex-coworkers Javier Martin-Artajo and Julien Grout for their roles in a 2012 trading debacle that cost the bank more than $6bn. But over the past year, Iksil changed his story.