Former England and Aston Villa defender Ugo Ehiogu has died at the age of 44 after suffering a cardiac arrest at Tottenham's training centre on Thursday.
A Spurs statement said Ehiogu died in hospital in the early hours of Friday.
Ehiogu, who was Spurs' Under-23s coach, was capped four times by England.
"Words cannot express the shock and sadness that we all feel at the club," said Tottenham's head of coaching and development John McDermott.
"Ugo's immense presence will be irreplaceable."
- 'A hugely popular figure with a growing reputation as a coach'
- Sportsday - reaction and tributes after Ehiogu dies
Ehiogu made over 200 appearances for Aston Villa between 1991 and 2000 and then spent seven years at Middlesbrough.
He won the League Cup with Villa in 1994 and 1996, and also with Boro in 2004.
The centre-back also played for West Brom, Leeds, Rangers and Sheffield United, before retiring in 2009. He began coaching at Tottenham in 2014.
Ehiogu was a co-founder of music label Dirty Hit, which has British indie band The 1975 on its books.
He married his wife, Gemma, in 2005. He had two children - son Obi Jackson and daughter Jodie.
Villa will hold a minute's applause before their Championship match against Birmingham City on Sunday, with both sets of players to wear black armbands.
The same tribute will be paid before Spurs' FA Cup semi-final against Chelsea at Wembley on Saturday.
Meanwhile, Spurs' reserve team game at Manchester United on Monday has been postponed.
And all of the club's weekend academy matches have also been called off.
'A tremendous person'
"I can't fathom he's no longer here," former Middlesbrough goalkeeper Mark Schwarzer, who won the League Cup with Ehiogu, told BBC Radio 5 live.
He added: "He was a tremendous person, a tremendous character, a dedicated footballer and dedicated to his family. He was a great guy to be around, so full of life and so enthusiastic.
"As a centre-back, I rate him right up there. He suffered from injuries throughout his career, but with more consistency he could have added to his England caps.
"He was very much a family man and it's such a shame to leave such a young family behind."
Former Aston Villa team-mate Andy Townsend told BBC Radio 5 live: "He was a defender every team would like to have at the back. It's a life that is so tragically cut short and so sad.
"He was on the training field with the academy boys and would have stayed active. I saw him recently and he was a picture of health, which is why this come as such a huge shock."
'One of our heroes'
Middlesbrough bought Ehiogu for a then club record fee of £8m in 2000 and he became a mainstay of the defence alongside Gareth Southgate as Steve McClaren's side won the League Cup.
"Ugo was one of our heroes at Cardiff when the club won its only ever major trophy," said Boro chairman Steve Gibson.
"Ugo and Gareth Southgate were the rock on which Steve McClaren brought the club its best period in its history. He wasn't just a good footballer, he was a great man."
Former Middlesbrough boss Bryan Robson, who signed Ehiogu, added: "He was such a good, strong defender and a fitness fanatic, which is why it becomes a real shock.
"I know he was a good lad and a team man who would chat to everybody, so I always thought he could be a coach because he was good at dealing with young lads."
Aston Villa manager Steve Bruce, who played for Manchester United in the 1994 League Cup final, added: "I had so much admiration for him as a fellow centre-half.
"He was uncompromising, quick and gave his all every single week - he was a great defender. All the football world will be saddened."
Former Villa boss Ron Atkinson, who brought Ehiogu to the club in 1991, said: "It is a complete shock. He was a big physical specimen, a strong man. You realise that can happen to anyone.
"He was a defender that liked defending, he loved a full-blooded challenge.
"He didn't have the best of starts for Villa and made a couple of blunders against Norwich that cost us the result, and it took him a long time to live that down. But he showed character and developed into a centre-half who would have got a lot of England caps but for injury."
Spurs manager Mauricio Pochettino added: "Ugo was a lovely man and we had a very good relationship. It's a huge loss personally and for all the Tottenham family."
Club chairman Daniel Levy said: "This is an incredibly sad day for the club and a tragic loss of a talented member of our Spurs family. Ugo was an extremely popular and respected academy coach, a tremendous influence on our younger players, both in training and away from the pitch."
Football Association chairman Greg Clarke said he was "a hugely popular figure across English football but particularly at Aston Villa and Middlesbrough. He was also close to many at Wembley and St George's Park through his England connections - both as a player and as a coach".
Tributes on social media
Ex-Spurs midfielder Jermaine Jenas tweeted: "Gutted is an understatement. An aspiring coach and all round top guy. My thoughts and prayers are with your family."
Former England women and Arsenal Ladies forward Kelly Smith on Twitter: "RIP my friend Ugo Ehiogu, gone too soon. A wonderful, caring man."
Ex-Blackburn, Chelsea and Celtic striker Chris Sutton posted: "Really upsetting news about Ugo Ehiogu. Football has lost a great player and a great man. Thoughts go out to his family. RIP Ugo."
Ex-Aston Villa goalkeeper Mark Bosnich tweeted: "RIP Ugo Ehiogu. Tremendous player and an even better man. Words can't do justice to how sad I am."
Former England defender Rio Ferdinand tweeted: "Can't believe the news that Ugo Ehiogu has passed away. Calm & warming vibe when in his company. My heart goes out to his family."
Ex-England defender Sol Campbell described Ehiogu as "one of my East London mates of old" and "a true defender". "My heart goes out to his family. I just can't believe it," he added.
Jamie Oborne, who co-founded Dirty Hit with Ehiogu in 2009, tweeted: "Gutted to hear that my friend Ugo has tragically passed away. I will always treasure the memories of our chats about love, life, hopes and dreams. Feel very blessed to have had you in my life. Love to Gemma and the little man. Such a sad day."
'A beast on the field, a gentle soul off it'
BBC Radio 5 live's Pat Murphy:
He was a very unassuming, gentle soul to deal with - very softly spoken. On the field he was an absolute beast, a colossus, an animal.
Gordon Cowans, a former midfielder at Aston Villa, and Ron Atkinson, his manager at the time, often tell the story of Ugo Ehiogu's first tackle at Villa.
In a pre-season game at Witney Town in Oxfordshire, he went into this tackle and Cowans, in the dugout with Atkinson, turned to him and said: "Did you see that?" He was such a strong defender, they knew they'd bought an absolute diamond for £40,000.
Atkinson knew him at Sheffield Wednesday and when he moved to Villa in 1991, he went after Ugo.
In his first game - Norwich City at home - he had a shocker. They lost 3-2 and Atkinson joked with Ugo that he was the player of the month - but he took it well.
For the next nine years, he was a tremendous centre-back alongside Gareth Southgate - a mixture of elegance, technical ability and strong, aggressive defending from Ehiogu.
Ehiogu was from the school of hard knocks. When he was trying to make his way in the game, he wrote to about 10 top clubs looking for a chance but got nowhere - other than at West Brom, where Atkinson liked the look of him.
So Ugo's career was a triumph of perseverance as well as undoubted talent and ability to look after himself physically.
He was a role model to a lot of the young players he'd been coaching at Tottenham because he could say he didn't have a gilded passage, like so many in academies nowadays.
Ehiogu had to work for everything he got, as an international and a Premier League defender.