The 27-year-old two-time World championship medal winner had had been due to compete in the men’s quadruple scull, which starts this weekend.
However, he fell ill on Saturday, a day after he arrived in Rio and has been forced to wuithdraw from the Games. He has been replaced in the team by 22-year-old Jack Beaumont.
A Team GB statement said Thomas had come down with a “flu-like virus understood to have been incubated before his arrival in Brazil” – effectively ruling out Zika – and had been withdrawn on medical advice.
Thomas, who has won silver and bronze medals at World Championships, said: “This is a response every athlete hopes they never have to write.
“Seven years of dedication and it’s all over before getting to take a stroke in anger. I don’t agree with the decision, but I don’t think anyone in my shoes would. As an athlete you always back yourself.
“I wish my friends and crew all the best in their Olympic regatta. I still believe they can do it without me and I hope all of this can lift any pressure or expectations and allow them to go out and enjoy it which is usually when people race best.”
Rowing team leader Sir David Tanner said the decision to withdraw Thomas had been taken with a “heavy heart”.