English soccer's most successful manager -- Manchester United's Alex Ferguson -- is retiring at the end of the season after more than a quarter. Former Manchester United goalkeeper Peter Schmeichel has described Sir Alex Ferguson's decision to retire as manager as a “bombshell”. Sir Alex Ferguson called it “the biggest mistake of my career”. His error was announcing in the summer of that he would retire at the end of that season. Fergie had already served 15 Video Loading. Video Unavailable. The long-serving Manchester United shocked football by announcing his retirement on 8 May He won 13 Premier League titles, five FA. On This Day In Sir Alex Ferguson Announced His Retirement. Tue May This video is either unavailable or not supported in this browser.