Britain's Olympic champion Duncan Scott, who won four medals at last year's Tokyo Games, pulled out of the swimming world championships on Tuesday due to the lingering effects of COVID-19.
The swimming events start in Budapest on Saturday and run until the 25th, with Scott's absence a big blow to Team GB's relay hopes.
"I’ve decided to pull out from the World champs that start this week. Absolutely devastated," Scott said on Twitter.
"Since returning from COVID I’ve been really struggling with high intensity. Now it’s time to put my health first and focus on recovery."
Scott, who revealed he had COVID on June 1, won gold in the 4x200-meter freestyle in Tokyo and silvers in 200 freestyle, 200 medley and 4x100 medley to become Britain's most-medaled athlete at a single Olympics.
The Scot is also Britain's most decorated Olympic swimmer after taking two relay silvers at Rio 2016 and currently holds the best time in the world this year in the 200-meter medley.
At the 2019 world championships in Gwangju, South Korea, bronze medalist Scott pointedly blanked winner Sun Yang of China, currently banned for doping violations, at the 200 freestyle medal ceremony.
"You loser, I’m winning, yes," Sun shouted at the Glasgow-born swimmer.
Britain will also be without eight-time world champion and triple-Olympic gold medalist Adam Peaty in Budapest after the breaststroke specialist fractured a bone in his foot in May.
Scott and Peaty will be Britain's main medal hopes for the Commonwealth Games in Birmingham where the swimming starts in July 28.
Several other top swimmers are also not defending titles in Hungary through injury, suspension, retirement or just deciding not to take part.