Ex-Societe Generale trader Jerome Kerviel, who is appealing a three-year prison sentence for his role in France's biggest-ever rogue trading scandal, is suing his former employer for allegedly obtaining a verdict under false pretences, his lawyer said on Friday.

SocGen, France's second-biggest listed bank, swiftly reacted by saying it would countersue for false accusation and would develop its position once Kerviel's appeal begins on June 4.

Kerviel -- who was also ordered in 2010 to pay 4.9 billion euros ($6.47 billion) to SocGen to make up for the losses that almost brought the French bank to its knees -- has never denied he lied to cover up his risky bets but has always claimed his superiors knew what he was doing.

We filed a complaint against Societe Generale today for obtaining a verdict under false pretences, David Koubbi, Kerviel's lawyer, told Europe 1 radio on Friday.

We hope the complaint will be favorably accepted.

SocGen said it acknowledged the filing of the complaint and said a countersuit would be filed in the briefest of delays.

Obtaining a verdict under false pretences means that a party has either provided false evidence or hidden contradictory information to get a favorable verdict, according to Paris-based lawyer Mabrouk Sassi.

It's a criminal charge and that gives additional powers to the judge such as the ability to order searches, he said.

We'll have to wait and see but maybe the lawyer is saying that SocGen has information that it wishes to keep secret that might clear Kerviel.

($1 = 0.7571 euros)

(Reporting by Matthieu Protard and Lionel Laurent)