PARIS - A FRENCH judge has opened an investigation into a possible breach of trust by a dealer suspected of losing a bank more than 750 million euros (S$1.42 million) in unauthorised trading in derivatives, judicial officials said on Thursday.

The dealer, 33-year-old Boris Picano-Nacci, has been in police custody since Wednesday morning and was transferred on Thursday evening to the headquarters of the division dealing with financial crimes after 36 hours of questioning.

He was banned from leaving the country or having any contact with former colleagues.

He was taken into custody as part of a preliminary investigation that seeks to determine whether anyone is criminally liable for the massive losses, racked up at the height of the global financial crisis, by the Caisse d'Epargne bank.

The bank announced losses of 600 million euros on Oct 17 and has revised the figure upwards several times. It now puts its losses at 751 million euros.

The traders reportedly exceeded limits set on the amount of funds that could be risked at any one time in market plays.

According to an internal report seen by AFP, the trader ignored instructions given in April to gradually unwind his market positions.

'Instead, from Sept 15... he appears to have engaged in a risky strategy, acting upon his own initiative and for major sums,' the report said.

'This strategy was renewed several times throughout the month and in October, including for extraordinarily high amounts,' but went undetected due to 'deficiencies' in the control system at Caisse d'Epargne, it said.

Finance Minister Christine Lagarde, furious the case undermined bids to restore confidence in France's financial sector, said a banking commission inquiry found 'serious deficiencies' in the bank's controls.

The scandal revived memories of the disaster at another French bank, Societe Generale, which lost 4.9 billion euros in unauthorised deals allegedly made by a junior trader Jerome Kerviel.

The French government has insisted the country's major banks are all stable despite the international credit crunch and have in any case promised state aid to bail out any failing institution and protect depositors.

News of the loss came in the same week as directors of Caisse d'Epargne approved plans to open merger talks with Banque Populaire to form France's second-largest retail bank.

Caisse d'Epargne has 27 million account holders - nearly half of all savers in France - and employs 51,500 people. -- AFP