The Enhanced Games is a "reckless" venture that could dissuade athletes of the future from competing in sport, says the athlete commission of UK Anti-Doping (Ukad).
The commission - a group of current and former athletes advising Ukad - believes the Enhanced Games could "damage the integrity of world sport irrevocably."
The controversial event, scheduled to take place for the first time in 2026, allows athletes to take banned performance-enhancing drugs under medical supervision.
It has been criticised for endangering athletes' health and undermining fair play, though organisers say there are safety measures in place, including supervision from doctors and healthcare professionals.
Last week, Olympic swimmer Ben Proud became the first British athlete to sign up.
"The games represent a reckless departure from what sport is all about, and its fundamental values of fairness and integrity," read an athletes commission statement.
"We believe that celebrating performances achieved through doping goes against everything the world of sport should be striving for.
"Sporting performance on the world stage should serve to inspire the next generation, and our fear is that this competition will intimidate, demotivate and in effect turn away potential athletes of the future who may think the only way to succeed is through cheating."