Blues strengthened their defensive ranks by signing former Southampton centre-half Aaron Martin in January 2014 on a short-term deal until the end season.
Martin began his footballing career with non-league side Eastleigh, before joining Southampton in November 2009.
During his time with the Saints, he made 29 first-team appearances and scored two goals.
The Isle of Wight-born stopper also spent time out on loan at Salisbury City, Crystal Palace and Coventry City.
He missed the first half of this season because of an injury that he picked up last summer but he is now back fully fit and turned out for the Saints' development squad earlier this month, in a game in which he also scored.