A UKIP MEP who spent three nights in hospital after a row with a party colleague is quitting the party, saying it is in a "death spiral".
Steven Woolfe, who had been running to be the next leader, told the BBC there was "something rotten" in the party.
He also accused fellow MEP Mike Hookem of inflicting a "blow" to his face in the row at a party meeting.
Mr Hookem has acknowledged a "scuffle" but said he "categorically did not" throw a punch at his colleague.
Mr Woolfe was rushed to hospital after collapsing following the incident, which UKIP described as an "altercation", and came during a meeting to discuss reports the North West England MEP had had discussions about joining the Conservatives.
'Man to man'
In the BBC interview, Mr Woolfe said he had told Mr Hookem "let's go outside and discuss this man-to-man" after they clashed during UKIP's meeting in the European Parliament, saying he had been suggesting they discuss their differences verbally.
Asked what happened next, Mr Woolfe said: "He rushed at me. A blow to my face forced me back through the door."
He added: "I couldn't see whether it was a fist, whether it was an open hand... the point was it was a blow that impacted me."
Mr Woolfe said he was pushed back into the room and hit the back of his head against a wall and denied aggression towards Mr Hookem, saying: "It was too quick."
He said that while he was voting in the European Parliament later, he started getting a severe headache and sought medical help.
He said the doctors were "incredibly concerned" about him and that he was unconscious "for quite some time".
Had it not been for the medics, he said "things would have been a lot more severe for me".
According to Mr Hookem, Mr Woolfe suggested they "take it outside of the room".
Speaking to the BBC after the incident, Mr Hookem said: "When I walked in he approached me to attack me. He came at me, I defended myself. There were no punches thrown, there was no face slapping, there were no digs, there was nothing."
Mr Woolfe was seen as a frontrunner to become UKIP leader after the surprise resignation of Diane James.
However, he said he was withdrawing as a candidate and resigning from the party immediately, "with a huge amount of sadness".
He said divisions within the party had created "huge negative camps", adding: "There is a spiral that is going on that's bringing it down."
There had been some "horrific" things said about him after the row, Mr Woolfe said, describing the episode as a "horror story".
He added: "I think they've got a spiral, someone suggested that it's a death spiral, of their own making."
Only a "small handful" of UKIP politicians and officials had contacted him to ask how he was since the incident, he added.
'Sadness and disappointment'
In a statement on his website, Mr Woolfe said that, contrary to reports, he had made a police complaint about the incident involving Mr Hookem.
He added that in his view, UKIP was "ungovernable without Nigel Farage leading it and the referendum cause to unite it".
Nominations to replace Ms James close on 31 October, with the new leader announced on 28 November.
One of the contenders, Raheem Kassam, said those responsible for "negatively campaigning" against Mr Woolfe should "hang their heads in shame" and urged him to return to UKIP.
UKIP chairman Paul Oakden said he felt "sadness and disappointment" at Mr Woolfe's resignation.
He said he disagreed with Mr Woolfe's characterisation of the party, and predicted the forthcoming leadership race would showcase the "strength and depth of talent" in the party.