* AMF proceedings started against 2 execs
* Jean-Pierre Mustier, Robert Day deny allegations
(Adds detail, background)
PARIS, Aug 6 (Reuters) - Jean-Pierre Mustier, a senior Societe Generale executive whose reputation was hit by last year's trading scandal, has resigned after the French market watchdog opened insider trading proceedings against him.
Societe Generale said on Thursday it had been told by Mustier -- head of investment banking at the time of the Jerome Kerviel trading scandal -- and by Robert Day, a non-executive director, that they had each been advised by the AMF regulator of insider trading proceedings against them.
Societe Generale said both men reject the allegations. The bank said it does not comment on ongoing individual procedures.
In January 2008, SocGen unveiled 4.9 billion euros of losses which it said were caused by unauthorised deals carried out by Kerviel, a junior trader at the bank.
Mustier, most recently head of the group's asset management arm as well as private banking and security services and a member of the executive committee, was once tipped as a future chief executive. He was replaced as head of corporate and investment banking after the Kerviel scandal.
He had been due to step down from the group when the merger of its asset management arm with that of Credit Agricole was complete, and by December 31 2009 at the latest, the bank said.
(Reporting by Helen Massy-Beresford and Sudip Kar-Gupta; Editing by David Cowell)