Neil Harris has stepped down from his role as manager of Millwall after four and a half years in charge.
The former Lions striker, the club's all-time leading scorer, cemented his status as a hero at the club with a successful period at the helm, winning promotion to the Championship in 2017 and twice reaching the FA Cup quarter-finals.
Chairman John Berylson said in a statement on millwallfc.co.uk: "Firstly I would like to place on record my sincere gratitude to Neil for all of his hard work, commitment, determination and inarguable success as manager of this great football club."
Harris was the seventh longest-serving manager across the Football League and Premier League and leaves with Millwall sitting 18th in the Championship table and without a win in seven matches.
Berylson's statement continued: "During his time in charge we have achieved so much and so many of his greatest moments as manager will live long in the memory of all Millwall fans fortunate to have seen and enjoyed them.
"Neil's achievements ensure that he is listed alongside some of the club's greatest-ever managers, furthering his legendary standing at Millwall."
Millwall came through the League One play-offs in 2017, beating Bradford 1-0 in the final. The same season they beat Bournemouth, Watford and Leicester to reach the last eight of the FA Cup before a heavy defeat by Tottenham.
Last season they also made the last eight, beating Everton but then losing to Brighton on penalties.
Harris' assistant David Livermore has also left the club, with first-team coach Adam Barrett placed in temporary charge. He will occupy the hotseat for Saturday's clash with Leeds.
Berylson added: "Neil's name is synonymous with Millwall and he has always had the very best interests of the club at heart, managing with great pride, honour and integrity, and he departs with the best wishes of everyone connected with The Lions.
"Both Neil and David leave with their heads held high, forever friends of the club, and I wish them both every success in their future careers. They will always be welcome at The Den."