Bradley Wiggins has pulled out of the Giro d'Italia because of a chest infection.
The Team Sky rider and Tour de France winner withdrew on medical advice ahead of today's 13th stage from Busseto to Cherasco.
Team medical staff had monitored the 33-year-old overnight after he struggled on stage 12 from Longarone to Treviso and was dropped by the peloton in the closing stages as fellow Briton Mark Cavendish won the stage.
Team Sky principal Dave Brailsford said: "We monitored Bradley overnight and this morning we've withdrawn him from the Giro after consulting the team doctor. His chest infection has been getting worse and our primary concern is always the health of our riders.
"Bradley will return to the UK today for treatment and to rest and we hope to have him back on the road as soon as possible.
"As a passionate racer he wanted to continue but he is simply unable to do so on medical grounds."
Team Sky will concentrate on supporting Colombian Rigoberto Uran who currently sits in third place overall, two minutes and four seconds off the race lead held by Vincenzo Nibali.
Wiggins had fallen five minutes and 22 seconds behind the lead but has now been forced to pull out on the same morning as defending champion Ryder Hesjedal, who has withdrawn because of an unspecified medical problem.
The Canadian, who rides for Team Garmin-Sharp, said: "It's heartbreaking. I want to be here for my team and for all the people who have supported me to get me here to this point.
"I built my entire season around the Giro and I came here feeling great, but I have been suffering since the TT (time trial). We're working on it, but we're not sure what's wrong.
"There's a virus that's been going around, so it could be that, or severe allergies, or going too deep on the TT combined with both - whatever it is, I'm only getting worse.
"Yesterday's stage was just too much for me, I fought to get through it and I know everyone suffered but after seeing the medical staff last night, I also know that it's time for me to go home, get some tests done and get healthy again."
Nibali goes into today's 254 kilometre stage with a 41-second overall advantage over Australian Cadel Evans, with Uran third.
Brailsford added: "Rigoberto did a great job and retained his third position overall. He's looked very strong this race and we'll now do everything to support him in the race for the general classification."