In a move which could alert Leeds United, Stockdale is preparing to quit St Andrews as Birmingham manager and former United head coach Garry Monk begins rebuilding his squad.
Stockdale, 32, joined Birmingham on a free transfer last summer having helped Brighton to promotion to the Premier League but rejected a new deal at The Amex Stadium.
Harry Redknapp, the manager who took him to Birmingham on a three-year contract, lost his job less than two months into this season and Stockdale was part of a side who battled against relegation for most of the Championship term, making 36 starts.
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Leeds, who are understood to have been alerted to Stockdale’s availability, intend to sign another keeper to compete with 21-year-old Bailey Peacock-Farrell next season.
Peacock-Farrell staked a claim with a convincing run of 10 outings towards the end of this season and won a call-up to the Northern Ireland squad on Wednesday but Leeds want to recruit another option following the failed signing of Felix Wiedwald.
United paid Â£500,000 to take Wiedwald from Werder Bremen last June but are ready to cut their losses just 12 months into Wiedwald’s three-year deal. The German struggled to adapt to the Championship and was dropped by both head coach Paul Heckingbottom and his predecessor, Thomas Christiansen.
The future of second-choice Andy Lonergan is also uncertain. Lonergan is under contract until 2019 but was on the verge of joining Sunderland in January before the move fell through on deadline day.