The favourite to take over as leader of Ukip has said he suffered two seizures after a fellow MEP landed a blow on him in the EU Parliament in Strasbourg.
The 49-year-old collapsed and underwent a brain scan following reports of a confrontation with Yorkshire and Humber MEP Mike Hookem at the European Parliament in Strasbourg.
However, a spokesman for 62-year-old Mr Hookem denied a physical fight took place, telling the MailOnline: “Mike did not touch him”.
Party leader Nigel Farage has ordered an inquiry in to the incident, which comes days after the shock resignation of newly-elected leader Diane James.
Mr Woolfe, 49, said in a statement that he was being detained in hospital overnight as a precautionary measure, adding: At the moment I am feeling brighter, happier, and smiling as ever.”
Mr Farage confirmed that tests had not found any bleeding on the brain but said the North West England MEP had suffered two seizures – including “one quite major one”.
“He did lose consciousness for a bit so things were pretty bad. He still has a bit of numbness down the left side of his face so it has been a pretty serious medical incident,” he told reporters.
The confrontation threatens to further damage the reputation of the party which is locked in a leadership crisis following the resignation of Ms James, just 18 days after she was elected to the post.
In a sign of the bitter divisions, the party’s millionaire backer Arron Banks demanded the suspension of the party’s ruling national executive committee.
He warned that he would leave altogether if the party’s “Tory troublemakers and fifth columnists” succeeded in preventing Mr Woolfe from running for leader for a second time.
Mr Woolfe was the first contender to throw his hat in the ring having been barred from standing in the last leadership election after it was ruled he submitted his nomination papers 17 minutes late.
There were claims that he had angered some MEPs by admitting that he had considered defecting to the Conservatives before declaring his candidacy.
Mr Farage refused to be drawn on who else was involved in the incident which he likened to the conduct “you see in third world parliaments”.
“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,” he said.
“You see third world parliaments where this sort of thing happens. It’s not good, it shouldn’t have happened.”
MEP Roger Helmer, who was at the meeting, confirmed there had been “a lively exchange of views” but said he did not see any physical confrontation.
He said the incident took place “a good two hours” before Mr Woolfe collapsed during a voting session at the parliament.
“There were some lively exchanges of views. I think you’ll find that’s not uncommon in political meetings. I certainly saw no physical altercation take place,” he told the BBC.
A photograph posted on the ITV News website showed Mr Woolfe lying spreadeagled and face down on a gangway clutching a briefcase.
A spokeswoman for French national police said that the incident had not been reported to them and they had no current plans to investigate.
Feelings were already running high in the party, with some members deeply unhappy at Mr Farage’s decision to carry on as interim leader until a permanent successor was in place.
But Mr Banks warned critics of Mr Farage not to prevent Mr Woolfe – widely seen as the leader’s preferred successor – from standing again.
He singled out the party’s only MP Douglas Carswell and the leader in the Welsh Assembly Neil Hamilton – both ex-Conservative MPs.
“The Tory troublemakers and fifth columnists represent a small minority in our party, yet they use any opportunity they can to undermine those working tirelessly to hold the Government’s feet to the flames. This ends today,” he said.
“If Neil Hamilton and Douglas Carswell remain in the party, and the NEC decide that Steven Woolfe cannot run for leader, I will be leaving Ukip.”