One of the favourites to take over from Nigel Farage as UKIP leader is in “serious condition” after collapsing following an altercation with a colleague.
The incident took place during what was said to have been a heated discussion at the European Parliament in Strasbourg.
There were unconfirmed reports that Mr Woolfe was punched during the course of an argument.
A photograph posted on the ITV News website showed the MEP lying spread-eagled and face-down on a gangway clutching a briefcase with an attendant leaning over him.
According to reports he has now regained consciousness and his condition is improving.
In a brief statement, party leader Nigel Farage said: “I deeply regret that, following an altercation that took place at a meeting of UKIP MEPs this morning, Steven Woolfe subsequently collapsed and was taken to hospital. His condition is serious.”
The incident happened a day after Mr Woolfe declared that he was a candidate for the leadership following the shock resignation of Diane James just 18 days after she was elected to the post.
According to one source, it took place at what was described as a “clear-the-air meeting” with a group of MEPs who had been unhappy at his admission that he had considered defecting to the Conservatives.
In his statement announcing his candidacy, Mr Woolfe said he had been “enthused” by Theresa May’s start to her premiership, prompting him “as it did many of my friends and colleagues, to wonder whether our future was within her new Conservative Party”.
Neil Hamilton, Ukip’s leader in the Welsh Assembly, said he had been told by an eyewitness that there was an argument between some MEPs.
The former Conservative MP told the BBC: “I don’t know what his current condition is. It’s obviously a serious matter if he’s been carted off to hospital with bleeding on the brain, as I understand.
“Well, this is what I’ve been told second-hand but the story has come from somebody who was an eyewitness there so you know I think it’s pretty credible.
“It’s most unfortunate but passions obviously run high.”
He added that he had been told Mr Woolfe “toppled over and hit his head on a glass window” after the confrontation, adding: “It must have been quite a wallop, I guess, to have had the impact that it seems to have had.”
Mr Hamilton admitted the incident was not a great advert for the party, but added: “Let’s not generalise it – it’s a dispute between one or two individuals. It’s not good that they are public representatives of the party in the European Parliament and that’s highly regrettable.
UKIP MEP Roger Helmer was also at the initial meeting, where he said there was “a lively exchange of views”.
He told the BBC: “I’ve heard that he is in a serious condition, although I’ve heard that my colleague Gill Seymour, who was with him, is optimistic about his recovery.
“There were some lively exchanges of views. I think you’ll find that’s not uncommon in political meetings.
“That took place a good two hours before the voting session where I understand Steven Woolfe left.
“I certainly saw no physical altercation take place.”
Mr Helmer added: “It was a closed meeting of UKIP MEPs, and as such it’s confidential so I’m not going to go into any details of what happened.
“We had a typical, general discussion about the previous leadership elections, the situation following Diane James stepping down, and obviously the steps we need to take to get a new leader in place.
“Those were the things you expect us to talk about, but as I say there were lively exchange of views.”
He described Mr Woolfe as “a very competent man”, adding: “We’re very concerned for him as a person, but also from the party’s point of view – I think we’d be extremely disappointed if he suddenly thinks he’s unable to stand.
“For both those reasons, we’re very much hoping for an early and complete recovery.”