Steven Woolfe (pictured above left), who suffered two seizures after an argument in Strasbourg, alleges Mike Hookem (pictured above right) "came at me and landed a blow" during a meeting intended to clear the air between the party's MEPs.
Mr Woolfe, 49, told the Daily Mail: "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."
Yorkshire and the Humber representative, Mr Hooken, 62, denied hitting his North West England colleague, telling the Daily Mail: "I did not hit Steven and I did not see him hit his head."
Interim Ukip leader Nigel Farage has announced a full investigation in the incident, which came one day after Mr Woolfe announced he would stand to take the helm of the party, following the shock resignation of Diane James on Tuesday.
Mr Farage likened the row to behaviour in "Third World parliament", adding: "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."
It has been claimed Mr Woolfe, who was barred from standing in September's UKIP leadership contest after he missed a paperwork deadline, angered some MEPs when he recently admitted he had pondered defective to the Conservatives.
He went on to tell the Daily Mail: "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."