United announced that the Scotland captain had been forced to take an indefinite break from playing after succumbing to a debilitating bowel condition.
Fletcher missed a large chunk of last season with a so-called 'virus' and then sat out much of pre-season when the problem resurfaced.
It is thought unlikely that he will play for the Barclays Premier League champions before the next season starts in September - and he may never return to the team at all.
His chronic illness has put a large question mark over the career of a man who captained Scotland at just 20 years old.Whether the midfielder can return will depend on the severity of the condition.
Olympic rower Sir Steve Redgrave won gold medals at five consecutive Olympics, despite being diagnosed in 1992, while England rugby star Lewis Moody also suffers from the illness.
A statement from Manchester United said: 'On behalf of Darren Fletcher, Manchester United wishes to announce that he is taking an extended break from football for health reasons.
'Over the past year he has had several absence periods which we have attributed to a viral illness in order to respect his right to medical confidentiality.
'Darren has, in fact, been suffering from ulcerative colitis (a chronic inflammatory bowel condition) for some time preceding this.
'While he was able to maintain remission of symptoms for a considerable period, this has proved more difficult recently and Darren's continued desire to play, and his loyalty to both club and country, has probably compromised the chances of optimising his own health and fitness.
'He has therefore accepted medical advice to take an extended break from the demands of training and playing in order to afford the best chance possible of achieving full remission once again.'
Fletcher's condition can be managed in some cases by drugs, diet and lifestyle changes. In other instances, though, it can't.
Some of the drugs available to him may also figure on the banned substances list for his sport.
In his autobiography, rugby star Moody explained about the difficulties of living with a similar illness.
He wrote: 'The condition will never go away completely. I am resigned to taking a minimum of four tablets a day for the rest of my life. On bad days it is eight tablets.
'I have special permission under the laws of the game - a therapeutic use exemption - which allows me to take them.
'I watch my diet - no more curries, red meat (especially lamb), brown bread (surprisingly), caffeine... even alcohol has to be taken in moderation.
It is a life sentence.'The news was greeted with dismay by fans of Manchester United, where Fletcher is one of the most popular players.
He appeared for the club on September 18, when they beat Chelsea, but has now not played since November 22.
It is understood he has not been training for a week.
Team-mates Rio Ferdinand and Michael Owen expressed their support on Twitter, as did former United goalkeeper Peter Schmeichel.
Fletcher does at least have some financial security. He was given a new four-year contract in March that takes him up to the summer of 2015 - something that now seems odd given how long his struggle with this illness has clearly been going on.
Source: Daily_Mail
