The French financial futures market, the Matif, has seen its level of business plummet since the switch to online trading and the launch of the rival Eurex exchange handling European financial futures in 1998. Eight major French banks last year got together to try to give the Matif a boost, and have since said business is picking up, notably recording a record level of trading on April 6.

But then in July press reports started surfacing of alleged fictitious transactions being recorded in an attempt to give the impression that the Matif was attracting more business than was in fact the case. The investigation opened Wednesday by the financial division of the prosecutor's office was sparked by the discovery of an internal memo during a search of the offices of the French stock market regulator, the COB, in June in relation to an enquiry into alleged insider trading by two COB staff.