Ukip's Steven Woolfe has claimed MEP Mike Hookem "came at me and landed a blow" in an altercation over the party's leadership contest.
Mr Woolfe told the Daily Mail that Mr Hookem lost his temper and pushed him into a door frame during a meeting designed to clear the air between Ukip's MEPs.
Mr Woolfe later collapsed and was taken to hospital in a "serious" condition, before feeling better.
Mr Hookem denied he hit his colleague and suggested the MEP for North West England tripped instead.
Mr Woolfe had been in a "serious" condition and is said to have had two epileptic-like fits.
However, he is now recovering in hospital following the incident at the European Parliament in Strasbourg on Thursday.
Mr Woolfe told the newspaper: "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.
"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."
Mr Hookem told the Daily Mail: "I did not hit Steven and I did not see him hit his head."
Mr Woolfe said he is "sitting up" and "and feeling "brighter, happier and smiling as ever".
A CT scan also showed there was no blood clot on his brain and he is being kept in hospital overnight for "secondary tests", he said.
In a statement, Nigel Farage said he "deeply regretted" the incident, which happened on Mr Woolfe's 49th birthday.
Suzanne Evans, Ukip's former deputy chair who is said to be considering her own leadership bid, tweeted:
It comes a day after Mr Woolfe put himself forward as a contender to be the next Ukip leader following the shock resignation of Diane James after just 18 days in charge.
He was barred from standing in the last leadership contest after submitting his nomination papers 17 minutes late.
His announcement came as Mr Farage confirmed he would be carrying on as interim leader until a permanent successor was in place.