UKIP leadership hopeful Steven Woolfe says he is recovering in hospital after a reported fight at a meeting of the party's MEPs.
The party released a statement from Mr Woolfe from his Strasbourg hospital bed saying he was sitting up having undergone a precautionary brain scan.
UKIP sources said "punches were exchanged" during the row at a party meeting and Mr Woolfe banged his head.
He was taken to hospital two hours later after collapsing, sources said.
UKIP sources said "a rumbustious argument" had taken place at the MEPs' meeting at the European Parliament over whether Mr Woolfe had been talking to the Conservative Party.
Even the possibility of him talking to the Conservatives had been seen by some as a betrayal and some MEPs were very angry, the sources said.
The BBC was told Mr Woolfe went outside with another MEP where "punches were exchanged" and it is believed that Mr Woolfe banged his head against a window or a wall but then got up.
Following a vote two hours later "he collapsed" and doctors were called and his wife was contacted.
UKIP did not deny Mike Hookem, who represents Yorkshire and North Lincolnshire, was the MEP who left the room with Mr Woolfe. Mr Hookem's spokesman reportedly told the MailOnline: "Mike did not touch him."
A party spokesman told the BBC the events that followed were not witnessed by MEPs.
Speaking from his hospital bed, Mr Woolfe said: "The CT scan has shown that there is no blood clot in the brain. At the moment I am feeling brighter, happier, and smiling as ever.
"As a precaution, I am being kept in overnight awaiting secondary tests to make sure everything [is] fine.
"I would like everyone to know that the parliamentary staff, the UKIP MEPs with me and hospital staff have been brilliant. Their care has been exceptional.
"I am sitting up and said to be looking well. The only consequence at the moment is a bit of numbness on the left hand side of my face. "
UKIP interim leader Nigel Farage, who is launching an inquiry into the altercation, said Mr Woolfe "is in a much better place than he was a few hours ago".
"He did collapse coming out of the chamber during the voting session and hit the ground pretty hard..." he said.
"It's been a pretty serious medical incident, but I'm pleased to say he's sitting up in bed and he's feeling a lot better."
UKIP chairman Paul Oakden told the BBC's Newsnight programme he was "in the process of trying to find out" what happened, and would take "appropriate action" if necessary.
He said it appeared it was a case of "two individuals who clearly got carried away" but it was not clear what caused Mr Woolfe to fall and it would be inappropriate to speculate.
Mr Oaken added: "The party is far bigger than any one individual. That's something we have demonstrated particularly over these last few months. While today's events were unfortunate, they don't characterise or define the party in any way."
Raheem Kassam, the other candidate to have declared for the UKIP leadership so far, tweeted his best wishes for Mr Woolfe, as did ex deputy chairwoman Suzanne Evans.
Mr Kassam cancelled his appearance on BBC2's Daily Politics "out of respect" for Mr Woolfe, following reports he had been taken ill.
He added: "I really hope @Steven-Woolfe is okay. Plz send him your best wishes."
Meanwhile UKIP financial backer Arron Banks has said the party is at "breaking point" and criticised UKIP's leader in Wales, Neil Hamilton, over comments he made in television interviews where he appeared to blame Mr Woolfe for events, "before anyone knew if Steven was going to be OK".
Mr Banks threatened to leave UKIP if Mr Woolfe was blocked from running for leader and if Mr Hamilton and UKIP's sole MP, Douglas Carswell, remained in the party.
Mr Carswell later said he would not "dignify" the "intemperate outburst" with a response, adding: "My thoughts are with Steven Woolfe."
BBC political correspondent Alex Forsyth said the row illustrated the split within UKIP which, in its broadest terms, was a division between Nigel Farage and those who supported his vision for the party and those who did not - including the former Conservative MPs Mr Carswell and Mr Hamilton.
Mr Farage said on Wednesday he would return as UKIP's interim leader until a fresh election could be held to find a successor to Diane James.
Ms James announced her resignation after just 18 days in the job, citing professional and personal reasons for her decision.
She succeeded Mr Farage on 16 September after he resigned following the Brexit vote.
Mr Woolfe was unable to take part in the previous UKIP leadership race after he missed the deadline for submitting his nomination.