British world heavyweight champion Tyson Fury has postponed his rematch with Wladimir Klitschko for a second time because he is "medically unfit".
Promoters did not disclose details but said "the condition is too severe to allow him to participate".
Fury, 28, beat Ukraine's Klitschko on points in the first bout between the pair in November last year.
The original rematch, scheduled for 9 July at Manchester Arena, was postponed in June after Fury injured his ankle.
The rearranged rematch - with the WBA and WBO belts on the line - had been scheduled for 29 October.
"Tyson will now immediately undergo the treatment he needs to make a full recovery," Hennessey Sports added.
"We and Tyson wish to express our sincerest apologies to all those concerned with the event and all the boxing fans who had been looking forward to the rematch. Tyson is understandably devastated by the development.
"We will be making a further statement in due course."
At a media conference before the initial scheduled rematch in April, Fury taunted Klitschko by telling him he was beaten by "a fat man" in their first encounter.
In August, Fury was charged with a doping offence by the UK's anti-doping body Ukad, with a hearing into the alleged violation to take place in November.
By Ben Dirs, BBC Sport:
"Few in the fight game will be surprised to hear that Fury-Klitschko II is off again.
"Last week, Fury failed to show up to a press conference in London, at which Klitschko said he suspected the rematch would never happen.
"Rumours have circulated that he has not been training and there is a real possibility that he will be stripped of his world titles, having not defended them since dethroning Klitschko last November."
The rocky road to Fury-Klitschko II
- 28 November 2015: Fury outpoints Klitschko in Dusseldorf to become heavyweight champion of the world.
- 2 December 2015: Klitschko announces he will take up his option of a rematch in the New Year.
- 16 December 2015: The BBC is forced to defend Fury's inclusion on the Sports Personality of the Year shortlist, following comments he made about women and gay people - he eventually finishes fourth in a public vote.
- 4 February 2016: Fury suggests that he has still not decided if he will fight Klitschko again and could quit boxing.
- 8 April 2016: A rematch is confirmed for Manchester on 9 July.
- 27 April 2016: At a press conference, Fury taunts Klitschko by telling him he was beaten by "a fat man" in their first encounter.
- 16 May 2016: Fury apologises after he is filmed making homophobic, sexist and anti-Semitic comments in a video posted online
- 24 June 2016: Fury announces the rematch will be postponed after he injures his ankle in training.
- 26 June 2016: A Sunday Mirror article reports that traces of a banned anabolic steroid were allegedly found in a sample given by Fury in 2015. The Briton denies doping.
- 7 July 2016: Fury states the new date for the rematch with Klitschko will be on 29 October.
- 4 August 2016: Fury is charged with a doping offence by the UK's anti-doping body. The 27-year-old says he will sue over claims he used a banned drug.
- 17 August 2016: Klitschko says he will take Fury to court to ensure their rematch takes place.
- 12 September 2016: Fury misses a pre-fight news conference in London because his car broke down on the way down from his home in Morecambe.
- 23 September 2016: The Klitschko v Fury rematch is postponed for a second time.