Ukip's Steven Woolfe has claimed a party colleague "came at me and landed a blow" in an argument over the party's leadership contest.
Leadership favourite Mr Woolfe accused MEP Mike Hookem of losing his temper and pushing him into a door frame during a meeting designed to clear the air between Ukip's MEPs.
Mr Woolfe, 49, is recovering in hospital after he collapsed and suffered two seizures following the incident at the European Parliament in Strasbourg on Thursday.
But Mr Hookem, 62, denied he hit his colleague and suggested the MEP for North West England tripped instead.
Mr Woolfe told the Daily Mail: "Mike was obviously very angry and lost his temper.
"I wasn't bruising for a scrap. I asked to deal with the matter outside of the room because it was flaring up in the meeting and upsetting everybody, and Mike clearly read that totally the wrong way.
"It was a completely unexpected incident.
"Mike came at me and landed a blow. The door frame took the biggest hit after I was shoved into it and I knew I'd taken a whack and was pretty shaken."
After the incident Mr Woolfe briefly attended a voting session at parliament but left when he felt unwell. Moments later he collapsed and was rushed to hospital.
Mr Hookem told the Daily Mail: "I did not hit Steven and I did not see him hit his head."
The incident threatens to further damage the reputation of the party, which is locked in a leadership crisis following the shock resignation of Diane James just 18 days after she was elected as leader.
Party leader Nigel Farage has ordered an inquiry into the confrontation.
Mr Woolfe had announced he will run for leader in the next election, after he was barred from standing in the leadership contest over the summer for missing the nomination deadline by 17 minutes.
But there were claims that he had angered some MEPs by recently admitting that he had considered defecting to the Conservatives.
Mr Woolfe added: "There was a lot of anger expressed towards me over what happened in the summer around the leadership contest, and the fallout on social media after I was barred from standing."
Mr Farage likened the incident to that you would see "in Third World parliaments".
"It's two grown men getting involved in an altercation. It's not very seemly behaviour, but I'm not today going to get involved in the blame game, name names and say who did what," he said.
"You see Third World parliaments where this sort of thing happens. It's not good, it shouldn't have happened."
In a sign of the bitter divisions, the party's millionaire backer Arron Banks demanded the suspension of the party's ruling national executive committee.
He warned that he would leave altogether if the party's "Tory troublemakers and fifth columnists" succeeded in preventing Mr Woolfe from running for leader for a second time.
Feelings were already running high in the party, with some members deeply unhappy at Mr Farage's decision to carry on as interim leader until a permanent successor was in place.
But Mr Banks warned critics of Mr Farage not to prevent Mr Woolfe – widely seen as the leader's preferred successor - from standing again.
He singled out the party's only MP Douglas Carswell and Ukip's leader in the Welsh Assembly Neil Hamilton – both ex-Conservative MPs.
"The Tory troublemakers and fifth columnists represent a small minority in our party, yet they use any opportunity they can to undermine those working tirelessly to hold the Government's feet to the flames. This ends today," he said.
"If Neil Hamilton and Douglas Carswell remain in the party, and the NEC decide that Steven Woolfe cannot run for leader, I will be leaving Ukip."