Mark Bowen has quit as Birmingham first-team coach with immediate effect.
Former Wales international Bowen joined the Blues with boss Steve Bruce in December 2001 and played a key role in their promotion to the Premiership.
A statement on Birmingham's official website said: "He has contributed significantly to the current success of the club.
"We shall be making an announcement about a new head coach when an appointment is made."
The statement added: "He is an excellent football coach with great ability and talent and he is passionate in his work.
"We are confident that his skills and working methods will enable him to continue coaching within professional football at the highest level.
"We thank Mark publicly for all his hard work and take the opportunity to wish him and his family well for the future."
Bowen's exit could open the door for him to return to the Wales set-up.