Steven Woolfe, a leading candidate to be the new leader of Britain's anti-EU UK Independence Party, is in hospital after suffering "epileptic-like" seizures following an "altercation" at a heated European Parliament meeting on the party's future.
Thursday's incident took place at a meeting of UKIP's European Parliament lawmakers, or MEPs, held to discuss what direction the party, riven by factional infighting since Britons voted to leave the European Union in June, should take.
The party was plunged into chaos on Wednesday when its leader Diane James quit just 18 days after being elected to replace the charismatic and well-known Nigel Farage who announced he would step down after the vote for Brexit.
Woolfe had angered some in his party when he said he would stand as a leadership candidate to replace James but then also admitted he had considered defecting to join new British Prime Minister Theresa May's ruling Conservatives.
"I deeply regret that following an altercation that took place at a meeting of UKIP MEPs this morning that Steven Woolfe subsequently collapsed and was taken to hospital," Farage, who has resumed his role as interim UKIP leader, said in a statement.
Woolfe collapsed and lost consciousness outside the EU legislature chamber and pictures showed him sprawled face down, still clutching a briefcase on a walkway in the parliament building containing MEPs' offices.
"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," Woolfe said in a statement from hospital. Scans had shown he had no blood clot on the brain but he was being kept in hospital overnight for more tests.
A UKIP spokesman said Woolfe, who was marking his 49th birthday on Thursday, had passed out and had two "epileptic-like fits" (seizures) but tests had shown no bleeding on the brain.
Roger Helmer, one of the UKIP MEPs at the meeting where the altercation occurred, said there had been "a lively exchange of views".
UKIP's Welsh leader Neil Hamilton, who was not at the meeting but said he had been given an account of what had happened by an eyewitness, said Woolfe had been knocked over and hit his head on a window.
"Steven I think picked a fight with one of them and came off worse," Hamilton told BBC TV. "It's most unfortunate but passions obviously run high."
French police and prosecutors in Strasbourg told Reuters that they were not investigating the case for the time being. A parliamentary official said the incident was being treated as an internal matter for UKIP and that no disciplinary procedure had been started.