Kenny Jackett has died at the age of 64.
"Millwall Football Club is extremely saddened to announce the passing of former manager Kenny Jackett at the age of 64," his former club confirmed on Friday.
"Kenny managed over 300 games during his time with the club, leading The Lions to promotion from League One and to an FA Cup Semi-Final.
"Rest in peace, Kenny."
Wolverhampton Wanderers, where he also managed, said: "Wolves are devastated to learn of the passing of our former manager Kenny Jackett.
"Kenny led the club to its incredible record-breaking League One title and laid the foundations for the club as we know it today.
"The thoughts of everyone at Wolves are with Kenny's family and friends at this time."
Swansea City said he was "responsible for starting the club on its remarkable rise from the bottom tier to the top flight."
They added added: "He will be deeply missed by all who knew him, but our club and the game of football as a whole has been all the better for his presence.
"He will always have a special place in the hearts of Swansea fans, and his place in the history of our club is not in doubt."
Jackett spent his 10-year playing career representing Watford between 1980 and 1990. He was capped 31 times by Wales.
As a manager, he began at Watford before moving to Swansea City. He most notably managed Millwall and Wolves. Jackett also managed Rotherham, Portsmouth and Leyton Orient.