The defender, who was entering the final year of his contract at Blues, had asked permission to leave for a Premier League club should an opportunity arise.
Davies established himself as a key performer at the heart of the Blues back-line during his two-and-a-half stay at St. Andrew’s.
His form last season saw with him pick up both the supporters’ and players’ player of the year awards.
The 28-year-old amassed a total of 104 appearances, scoring 12 goals.
Everyone at the club would like to thank Curtis for his service and professionalism and he leaves with our very best wishes.