A 24-year-old assistant of French rogue trader Jérôme Kerviel has been placed under formal investigation in connection with the loss of almost €5 billion at Societe Generale, the French bank.

Thomas Mougard, a trading room assistant, was accused of 'complicity in introducing fraudulent data into a computer system.'

Mr Mougard is alleged to have helped Jérôme Kerviel, the trader at the centre of the scandal, to input fictitious deals to hide illegal transactions.

The move is likely to be seized upon by Mr Kerviel's legal team as evidence to back his claim that bank staff turned a blind eye to his operations as long as he was making a profit.

The trader is alleged to have exceeded his authorised ceiling as he amassed hidden positions totalling almost €50 billion, which he then hid from his superiors.

When these were unwound in January, the bank emerged with a loss of €4.9 billion.

Mr Kerviel, 31, has been placed under investigation on suspicion of breach of trust, fabricating documents and illegally accessing computers.

He told investigators Mr Mougard had typed data into the bank's system knowing that it was fictitious in an attempt to help him cover up his tracks.

Mr Mougard denies the claims. He says Mr Kerviel told him the fake operations were designed only to disguise out-of-hours transactions.

Mr Kerviel has recently changed his lawyers to adopt a more offensive legal strategy.

Mairic Dupond-Moretti, one of his new lawyers, yesterday accused Société Générale of 'exploiting the penal justice system to designate someone as guilty.'

He said the bank wanted to escape criticism and legal action from its 'customers, public opinion and the American markets.'

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