Ukip's Steven Woolfe hit his head on a wall during an altercation with fellow MEP Mike Hookem and collapsed shortly afterwards, according to ITV News Political Editor Robert Peston.
Passions were high at a meeting of Ukip MEPs but Mr Woolfe decided against leaving the party, Peston told News at Ten.
"He was called effectively a traitor by Mr Hookem and they decided to take it outside," he said.
"There appears to be some dispute, doubt over whether punches were thrown but everyone says there was an altercation, some kind of a scuffle, he hits his head on the wall and later he collapses."
ITV News' political correspondent Carl Dinnen says a spokesperson for Mr Hookem told him it was "a purely verbal altercation" and no punches were thrown.
Sources told ITV News that "bruises consistent with a punch" were found by a team of independent experts.
Mike Hookem insisted he acted in self-defence during a "scuffle" with Ukip leadership favourite Steven Woolfe.
As Ukip and the European Parliament investigate the 'altercation' between Steven Woolfe and Mike Hookem, what could the repercussions be?