Kenny Jackett, who managed seven teams in the EFL, has passed away at the age of 64.
Jackett played over 400 games for Watford across his career and is ranked sixth in the club's all-time appearance record.
He achieved three promotions as a manager with Swansea City, Millwall and Wolves.
Jackett managed Watford, Swansea City, Millwall, Wolves, Rotherham United, Portsmouth, and Leyton Orient in a managerial career lasting over 900 games.
Clubs have been paying tribute to Jackett on social media.
His former side, Millwall, wrote: "Millwall Football Club is extremely saddened to announce the passing of former manager Kenny Jackett at the age of 64.
"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."
Portsmouth paid their tributes, noting: "Portsmouth Football Club are deeply saddened to learn of the death of Kenny Jackett, who passed away yesterday at the age of 64.
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Kenny Jackett has passed away at the age of 64
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"Kenny will be missed by everyone at Pompey and throughout the football family. Our thoughts are with all his loved ones at this sad time."
Wolves also paid their respects, writing online: "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."
Clubs have been paying tribute to Jackett
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The League Managers' Association chief executive Richard Bevan noted Jackett's "humility, professionalism and a deep care" for players and staff.
Bevan wrote: "Kenny stands as one of the most respected managers to have plied their trade in the EFL, a hugely capable leader whose work across four decades has left a lasting influence on the many players, coaches and colleagues who benefitted from his guidance.
"He embodied everything we like to see in a manager, humility, professionalism and a deep care for his players and staff. He improved every club he served and did so with quiet dignity throughout his career.
"From his first role at Watford to his final role at Gillingham, Kenny's work was characterised by an incredible sense of responsibility to his teams and to the game itself. Kenny is a huge loss to all that knew and loved him.
Jackett achieved three promotions during his managerial career
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"Our thoughts are with his wife Samantha, sons David and Ryan, and all of his family and friends, who join us in mourning."
Watford's chairman & CEO Scott Duxbury said: "There is a deep and profound sense of loss at the football club following the sad news that Kenny Jackett has passed away.
"Kenny holds legend status here following his remarkable achievements as a player, a coach and a manager, and the club has truly lost one of its own.
"On behalf of everyone at Watford FC, I send my sincere condolences to his wife, Samantha and sons, David and Ryan, the latter of whom is working with us today, building on the legacy his father left."
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