Ronnie O'Sullivan, the world champion, has called a time-out on his career in a move that casts doubt on whether he will play again.

The 36-year-old wrote to Barry Hearn, chairman of World Snooker, to confirm what he told the veteran sports promoter in a telephone call last week: that his season is over before it has properly begun.

A short statement from the organisation said: "World Snooker has been informed that Ronnie O'Sullivan does not intend to compete on the World Snooker tour for the remainder of the season."

The precise reasons were not disclosed, although it is known O'Sullivan has battled depression and illness recently. Django Fung, his manager, stressed there were a raft of causes for O'Sullivan deciding to step away from the sport.

O'Sullivan has played just one match in competition since landing his fourth world title at the Crucible on May 7, losing to the then world No.76 Simon Bedford at a minor tournament in Gloucester at the start of September.

That may prove to be the last tour match the Londoner plays, but Hearn hopes not. "It's difficult to say. Ricky Hatton has come back after three years in the wilderness, so you can never tell," Hearn said. "I've known Ronnie since he was 12 and I'd like to see him back to the bubbly character he used to be. He needs total time away from the game. It's a good decision he's made. He's got a lot of issues to deal with and he's not particularly well. He wants to take a complete break and see how he feels.

"I'm very relaxed about it. He's got a lot on his mind, and of course he's retired more times than Sinatra. It's a sensible, mature call to say, 'I can't just play at it'. It just piles up on you, and it probably feels it's a release."

Should O'Sullivan commit to next season, he would almost certainly find himself outside the world's top 32, needing to win at least two qualifying matches in empty snooker halls just to reach the venue stage of tournaments.

Since he beat against Ali Carter in Sheffield six months ago, O'Sullivan has fallen from ninth in the rankings to 20th, because of his inactivity, and with the season still in its infancy – the UK Championship takes place in York next month – he will plummet further.

The announcement didn't come entirely out of the blue. Prior to last week's Inter-national Championship in Chengdu, which he made a late decision to miss, citing illness, O'Sullivan said he was unsure whether he had made the right decision to enrol for another season.

He had committed to next month's UK Championship in York and January's Masters at Alexandra Palace, and was then expected to defend his world title at the end of the season.

But that all changes now, and for the time being at least the sport will have to cope without its biggest name, barely six months after losing another when seven-time world champion Stephen Hendry retired.