Sir Bradley Wiggins has withdrawn from the Giro d'Italia because of illness.
Seven-time Olympic medallist Wiggins, 33, has been suffering
from a chest infection.
The Team Sky rider will be aiming to recover in time to defend the Tour de France, which starts on 29 June.
"It's really disappointing to stop in this way because we came here for so much more," said Wiggins, who was five minutes and 22 seconds down overall after stage 12.
"When selecting his generals, Napoleon asked only one thing: 'Is he lucky?' He would have been deeply unimpressed with Bradley Wiggins's efforts to conquer Italy.
"That is not intended as a criticism of Britain's most decorated cyclist. It is merely a comment on how important fortune is in sport.
"We all like to think we are masters of our own destiny, but when you are hit with a chest infection, crashes and an untimely puncture, you just have to accept it's not your war."
"The decision was made with a view to being back to full strength for the Tour, so I think had we continued in this Giro, the risk was that I did more damage long-term."
Team Sky boss Sir Dave Brailsford added: "His chest infection has been getting worse and our primary concern is always the health of our riders.
"Bradley will return to the UK for treatment and to rest, and we hope to have him back on the road as soon as possible."
The Giro d'Italia was a target for Wiggins in 2013 as he sought to become the first British winner of the pink jersey.
It took on even greater importance after Team Sky said team-mate
Chris Froome would lead the British outfit's challenge
at the Tour de France.
Wiggins was one of the favourites for the road race in Italy after winning the Tour de France and Olympic time trial gold in 2012.
"As a passionate racer he wanted to continue but he is simply unable to do so on medical grounds," added Brailsford.
Wiggins endured a difficult Giro, falling in wet conditions on
and failing to win in his specialist event - the individual time trial - on
The Lancashire-based rider came home with his main rivals in stage 11, but was unable to respond when they quickened towards the end of stage 12, which was won by fellow Briton Mark Cavendish.
Wiggins was 13th in the Giro general classification, three minutes and 18 seconds behind Team Sky team-mate Rigoberto Uran in third, with leader Vincenzo Nibali a further two minutes and four seconds ahead.
Overall standings after stage 12
1. Vincenzo Nibali (Ita/Astana) 46:28:14"
2. Cadel Evans (Aus/BMC Racing) +41"
3. Rigoberto Uran (Col/Team Sky) +2'04"
4. Robert Gesink (Ned/Blanco) +2'12"
5. Michele Scarponi (Ita/Lampre) +2'13"
Despite Wiggins's exit, Brailsford insisted Team Sky are still in a strong position in the Giro, with Uran well placed.
"I think we have got a very strong team," said Brailsford. "They are all motivated and they will continue to ride for Rigoberto, so the team doesn't lose its purpose.
Bradley broke his collarbone in the 2011 Tour,
that was quite a difficult thing to take on because the team swapped from trying to get a result in the race to getting stage results, whereas here we still feel we are competitive."
Defending champion Ryder Hesjedal of Team Garmin-Sharp has also withdrawn from the Giro ahead of stage 13.
"The conditions on Thursday's rainy, cold stage combined with Hesjedal's deteriorating physical condition proved too much," the team said in a statement.
"It's heartbreaking," added Hesjedal. "I built my entire season around the Giro and I came here feeling great, but I have been suffering since the time trial."