Former UK Athletics performance director Neil Black, who defended Mo Farah's involvement with now-banned coach Alberto Salazar, has died at the age of 60, the governing body announced on Tuesday.
Black left his role last year after US authorities handed down a four-year doping ban to Salazar, who ran the Nike-backed Oregon Project training group in Portland.
Salazar worked closely with British distance runner Farah, who won four Olympic gold medals on the track, two at London 2012 and two more at Rio four years later.
A British Athletics statement read: "British Athletics is shocked and saddened to confirm the loss of our friend and former colleague Neil Black, who passed away suddenly at the weekend.
"Neil loved the sport of athletics and dedicated his life to supporting athletes - as a world-class physiotherapist, as head of sport science, and then in recent years as performance director for British Athletics."
Black was in charge when UK Athletics appointed Salazar, whom he once described as a "genius", as a consultant to its endurance programme in 2013.
The United States Anti-Doping Agency (USADA) began investigating Salazar in 2015 but UK Athletics reviewed his link with Farah at the time and concluded there was "no reason to be concerned".
There is no suggestion of wrongdoing from Black or Farah, who also has six world championship golds at 5 000m and 10 000m.
Black was a physiotherapist at UK Athletics before moving up the ranks to take the top job.
He took up the UK Athletics performance director role in 2012 after Charles van Commenee left his position as Olympic head coach.
Black stood down shortly after last year's World Championships in Doha, where Great Britain won just five medals, their worst performance since 2005.
Former Olympic heptathlon bronze medallist Kelly Sotherton, who worked with Black, tweeted: "He came everywhere with me and made sure I was held together physically and mentally. So many things I want to say... I'll miss you forever Neil Black."
Former sprinter Katharine Merry, who won bronze in the 400m at the 2000 Sydney Olympics, tweeted: "The passing of Neil Black is truly a shock to the world of athletics."
A tweet on the official Team GB account read: "Everyone at the British Olympic Association is shocked and deeply saddened by the news of the loss of Neil Black.
"Our thoughts are with his family, friends and colleagues at this extremely difficult time."