Tottenham's under-23 coach Ugo Ehiogu has died at the age of 44, the Premier League club announced on Friday.
Tottenham announced in a statement: "It is with immense sadness that we announce the passing of Ugo Ehiogu."
The former defender, capped four times by England, was being treated in hospital after collapsing at the club's training headquarters in north London on Thursday.
Ehiogu, who starred for Aston Villa and Middlesbrough duing his playing career, has been on Tottenham's coaching staff since 2014.
Tottenham's head of coaching and player development John McDermott said: "Words cannot express the shock and sadness that we all feel at the Club.
"Ugo's immense presence will be irreplaceable. Our heartfelt sympathies go out to his wife Gemma and his family."
The club's statement added: "Ugo received immediate treatment after suffering a cardiac arrest at the Training Centre yesterday (Thursday) before being transferred to hospital, where he passed away in the early hours of this morning."
Ehiogu made over 300 appearances in a nine-year stay with Villa, helping them win the League Cup in 1994 and 1996.
He joined Middlesbrough in 2000, spending seven years there and winning the League Cup again in 2004.
Ehiogu also played for West Bromwich Albion, Leeds, Glasgow Rangers and Sheffield United before retiring in 2009.
Villa reacted to the news on Twitter.
"We are deeply saddened to hear of the death of our former defender Ugo Ehiogu. Our thoughts are with his family at this terribly sad time."
The Championship club says it will hold a minute's applause at their derby match against Birmingham on Monday, with both sets of players wearing black armbands as a mark of respect to their former star.
Tributes to Ehiogu poured in from across the football world.
Australia's Mark Schwarzer, who played with the centre-back at Middlesbrough, said: "He was the ultimate professional and someone I really enjoyed playing with."
Former Manchester United star Rio Ferdinand tweeted: "Can't believe the news that Ugo Ehiogu has passed away. Calm and warming vibe when in his company. My heart goes out to his family."
Alan Shearer, the former England striker, wrote on Twitter: "Lost for words. A true gentleman. RIPUgo."
Former Republic of Ireland midfielder Andy Townsend, a Villa team-mate of Ehiogu's, added: "I'm absolutely shocked. I didn't expect to wake up to this news.
"He was a player that every team would like to have at the back.
"I remember the 3-0 win over Leeds (in the League Cup final). I looked at pictures at home this morning and Ugo is in them celebrating."