The US government’s case against two former JP Morgan staff embroiled in the $6.2bn ‘London Whale’ losses is at risk of collapse, with the key witness Bruno Iksil facing questions over a book he has written about the saga.

Lawyers defending Iksil’s boss, Javier Martin-Artajo, and the team’s junior trader, Julien Grout, plan to file a motion asking former JP Morgan trader Iksil to be questioned over a 400-page book that disagrees with the Securities and Exchange Commission’s version of events, according to a person familiar with the matter.