Barker, who would have turned 40 in March, started his career at Alfreton Town but made his professional breakthrough at Barnsley before signing for Cardiff City.
He went on to make a total of 675 professional appearances including spells at Stoke, Colchester, QPR, Plymouth and Southend before hanging up his boots at Weston-super-Mare.
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Having transitioned into coaching towards the end of his playing career, Barker had been working as Under-18s manager at League Two side Forest Green.
"We're all heartbroken to hear this news which came as a shock to everybody," said Hannah Dingley, Forest Green head of academy.
"Our immediate thoughts are with his family and friends.
"Barks was a strong, inspirational leader to our under-18s and a much-loved colleague.
"We'll be offering help and support to those at the club affected by the tragic news - particularly our young players, and ask that Chris' family get some privacy at this challenging time."
A number of Barker's former teammates were also quick to express their grief and pay tribute to his career within football and beyond.
Joe Ledley, who played with Barker at Cardiff, wrote: "Devastating news to hear the loss of Chris Barker my thoughts are with his family and friends, One of the most down to earth guys I’ve met. Will be truly missed. Rest In Peace barks."
Adam Webster, now of Brighton, played alongside Barker at Aldershot Town.
"RIP Chris Barker," he said on Twitter.
"A former team mate of mine and someone who helped me grow up on and off the pitch [at Aldershot] thoughts and prayers go out to his family. RIP barks."
Opponents and teammates alike made it clear that Barker had been a popular figure throughout his playing career.
Journeyman striker James Constable said: "Very sad to hear of the passing of Chris Barker, played against him so many times.
"A tough, no nonsense defender, but always enjoyed our battles.
"Thoughts and prayers are with all his friends and family."
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