Forensic accounting experts and rival banking executives are queuing up to question Société Générale’s assertion that a rogue trader could single-handedly hide more than €1bn ($1.5bn) of losses from the world’s leading equity derivatives bank.

A rival investment banking executive said it was “inconceivable” that Jérôme Kerviel could, as SocGen says, have built up positions worth about €50bn – more than its own market value – in futures for European equity indices without it triggering large and obvious margin calls (requirements for the bank to put up cash as collateral).