European Parliament President Martin Schulz has referred MEPs Steven Woolfe and Mike Hookem to French police after their “altercation” in Strasbourg earlier this month.
Mr Woolfe was left in hospital after the incident, and subsequently abandoned his UKIP leadership bid and quit the party.
Martin Schulz told the parliament on Wednesday he had referred the “regrettable” incident to French authorities “given the seriousness of the reported facts and their possible criminal implications”.
He told MEPs: “[The advisory committee on conduct] concluded that the versions of the facts given by the two members involved diverged substantially and the facts seem to have happened in the absence of direct witnesses.
“It also stressed that given the seriousness of the reported facts and their possible criminal implications, further evidence is needed to clarify this matter.
“As a result I have decided to follow the recommendation of the advisory committee and I have referred this matter to the competent French authorities.
“Based on the result of the investigations I will then take a decision about a sanction to be imposed.”
However, giving his own view, Mr Schulz said he was in “no doubt” Mr Woolfe was punched, and said medical examinations suggest he collapsed due to a blow to the head.
Mr Woolfe claims he “received a blow from Mr Hookem that knocked me back into the meeting room and caused my subsequent injuries” – something Mr Hookem denies.
In his resignation letter he added that made a police complaint and was “now seeking legal advice in respect of the investigations and will not be commenting further on the matter until the completion of those investigations.”