Chesterfield have confirmed that manager Paul Cook has been released from his contract to join Portsmouth.
The 48-year-old has spent nearly three seasons at the Proact Stadium, guiding the Spireites to the League Two title and League One playoffs in that time.
Talks are ongoing between Chesterfield and Pompey over assistant coach Leam Richardson's potential move to Fratton Park.
Cook's former clubs include Southport, Sligo Rovers and Accrington Stanley.