Former Cobblers Justin Edinburgh has died at the age of 49.
Leyton Orient boss Edinburgh, who managed the Cobblers for eight months in 2017, was taken to hospital on Monday after a suffering a cardiac arrest.
In a statement on the Os website, chairman Nigel Travis said: "We are completely heartbroken by this tragedy.
"All our thoughts and love are with the Edinburgh family and we know from the messages that have flooded into the club over the last week that the wider football world will share our sentiments.
"The success that Justin brought to Leyton Orient was incredible, but more importantly the impact he had on us all as a winner and a wonderful, inspirational human being will be his legacy and will stay with us forever."
Edinburgh, who won the FA Cup as a player with Tottenham Hotspur and also played for Southend United and Portsmouth, led Orient to the National League title this season as well as to the FA Trophy final.
The Cobblers released a statement which read: "Everyone at Northampton Town Football Club is shocked and deeply saddened to learn of the passing of former manager Justin Edinburgh.
"Justin had already achieved managerial success in Northamptonshire with Rushden & Diamonds when he joined the Cobblers in January 2017.
"He guided the club to some important victories in the second half of the 2016/17 season helping to secure League 1 status for another season, a run of six wins and a draw in ten games between January and March achieving safety.
"Justin left the club in late August 2017 and moved on to Leyton Orient three months later, leading the O's back to League 2 in the 2018/19 season.
"All of us at Northampton Town send our deepest sympathies and condolences to all of Justin's family and friends."
Town chairman Kelvin Thomas said: "“We are all saddened to learn of Justin’s passing.
"This is a huge shock for everyone and we extend our sympathy and condolences to Kerri, Charlie, Cydnie and all of the Edinburgh family.
“Justin was such a genuine, nice guy and someone who it was a pleasure to know. He did a good job for us, coming in during a difficult time and helping us to stay up in our first season in league one.
“We were delighted with the success he had at Leyton Orient and I spoke to him just after the end of the season to offer our congratulations.
“This is such sad news, it reminds us all how fragile life can be and we do anything we can to help and support all those affected by Justin’s passing.”
Edinburgh took over as Cobblers in January, 2017, following the sacking of Rob Page.
He was handed the task of securing the club's status in Sky Bet League One, and he did that successfully.
After a summer of heavy investment, the team started the following season poorly, and Edinburgh was relieved of his duties on August 31, to be replaced by Jimmy Floyd Hasselbaink.
He was soon back in work though, taking over at Brisbane Road, and in his first season he guided the club away from relegation trouble at the foot of the National League.
Last season he guided the west London club to the National League title and promotion back to the Football League, as well as reaching the FA Trophy final.
Chronicle & Echo sports editor Jeremy Casey said: "During his time as the manager of the Cobblers, Justin was a pleasure to deal with.
"He was always available, always happy to answer the phone and have a chat, and always willing to answer any question put his way.
"No matter how the team was doing on the pitch, or what pressure he may have been under or felt, Justin always met with the media, and more often than not, he would have a smile on his face.
"As in life in general, there are all sorts of characters in the world of football, but there is no doubt Justin was genuinely one of the nice guys."
People took to social media in their 1,000s to pay their respects and offer their condolences.
Edinburgh's former Tottenham team-mate Gary Lineker said: "Deeply saddened to hear that Justin Edinburgh has passed away. He was an excellent coach and a terrific fullback who was a delight to share a dressing room with and have as a teammate. Thoughts are with his family. #RipJustin"
Tottenham Hotspur tweeted: "We are deeply saddened and shocked to hear of the passing of Justin Edinburgh. Our heartfelt condolences and thoughts go out to his family and friends at this terribly difficult time as well as everyone that worked alongside Justin at Leyton Orient."
Another of his former clubs, Southend United, said: "The thoughts and heartfelt condolences of everyone at Southend United Football Club are with Justin Edinburgh’s family and friends and everyone at @leytonorientfc during this incredibly difficult time. RIP Justin."
The Cobblers' county neighbours AFC Rushden & Diamonds also tweeted: "Our thoughts are with Justin Edinburgh’s family at this time, he was a wonderful man and will be sadly missed by everyone."
Cobblers players past and present also spoke of their shock.
Junior Morias tweeted; "Absolutely gutted with the tragic news about Justin, thoughts are with the Edinburgh family and everyone at @leytonorientfc at this terrible time heaven as gained another angel."
David Buchanan said: "Lost for words how devastated i am. My thoughts are with all of Justin Edinburgh’s nearest and dearest, and @leytonorientfc. RIP boss."
Matt Crooks, who was signed by Edinburgh, said: "Life’s too short. Rest easy Justin Edinburgh."