The world No 3 was due to begin the defence of his title against John Read at the Bournemouth International Centre yesterday, but his manager, Ian Doyle, said: "Ronnie is suffering from physical and nervous exhaustion and his doctor has ordered a complete rest from snooker.
"The lad is desperately disappointed that he won't be able to defend his title. But at the moment Ronnie's health must come first."
O'Sullivan, who celebrates his 23rd birthday next month, was third favourite behind John Higgins and Stephen Hendry to take the pounds 75,000 on offer for winning the tournament. Read arrived at the BIC in the morning unaware that he had received a bye into the last 32.
O'Sullivan's withdrawal does not come as a total surprise to snooker watchers this season. Despite winning the Regal Masters title in Motherwell last month he has complained he had fallen out of love with the game.
"If I never saw a snooker table again it wouldn't bother me," he announced. "I hate the game. I'm just in it really for the money."
Even a few days before the UK Championship started O'Sullivan was still troubled. Asked to predict the score of his first-round match, O'Sullivan said Read would win 9-0 or 9-1. He went on to name the Canadian former world champion, Cliff "Grinder" Thorburn, as his role model.
O'Sullivan has been prone to mood swings throughout his career. At the 1996 World Championship at the Crucible Theatre, Sheffield, he assaulted a press officer, for which he was fined pounds 20,000 and given a suspended two-year ban.
He stayed out of trouble until earlier this season when he tested positive for cannabis during the Benson and Hedges Irish Masters.
O'Sullivan, who won the title by beating the local favourite, Ken Doherty, later had to forfeit his pounds 61,000 first prize though no ban was implemented.Reuse content