The favourite to lead the anti-EU UK Independence Party, Steven Woolfe, was in a "serious" condition in hospital on Thursday after an "altercation" at a meeting with colleagues in the European Parliament, former chief Nigel Farage said.
The incident at the parliament in Strasbourg, France, came two days into a leadership contest sparked by UKIP's new leader Diane James's resignation after just 18 days as Farage's successor.
"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 said in a statement.
"His condition is serious."
British media reported that Woolfe had been punched by a colleague and was being treated for bleeding on the brain but there was no immediate confirmation from the party.
A picture tweeted by Britain's ITV broadcaster showed a man in a suit said to be Woolfe sprawled face down on a walkway at the glass-and-steel parliament building, with a bag and a coat next to him.
The incident happened on Woolfe's 49th birthday.
A UKIP spokesman said Woolfe "was taken suddenly ill in the European Parliament building in Strasbourg this morning. He has been taken to hospital in the city and he is undergoing tests."
European Parliament spokesman Jaume Duch told AFP that Woolfe "fell ill in a corridor at the parliament near the hemicycle (main chamber). He was taken to hospital."
Asked whether there had been a fight, he said "we do not have that information".
The party has faced an existential crisis since pushing Britain towards a referendum on 23 June in which the country voted to quit the EU, and since Farage's resignation shortly afterwards.
James only became leader after Woolfe, her main rival, was dramatically ruled out of the contest after failing to submit his application in time.
Woolfe, who had been supported by both Farage and Arron Banks, the party's main financial backer, was quick to throw his hat into the ring to replace James on Wednesday.
Humble begin innings
Steven Woolfe, the hot favourite to lead the UK Independence Party, rose from tough beginnings to become a lawyer and one of the protagonists behind the successful push for Brexit.
Woolfe, who is of mixed race and hails from Manchester in northwest England, was seen within the anti-EU, anti-mass immigration party as an articulate spokesman who could broaden its appeal in working class areas beyond its southern English heartlands.
Born on October 6, 1967, Woolfe grew up in public housing in the Manchester suburb of Burnage.
He was the eldest of a family of four.
His brother Nathan Woolfe, 28, played professional football for Bolton Wanderers and Stockport County.
Woolfe's grandfather was African American and his grandmother was Jewish on his father's side.
On his mother's side, his grandfather was English and his grandmother was Irish.
"Today this mix is old hat, nothing unusual, pretty normal — and so it should be," he once said in a speech.
"Back in 1975, being mixed race... living in a council house... I was very much becoming aware of what it was like to be young boy growing up in a deprived part of England," he said.
Woolfe went to a predominantly Irish Catholic primary school.
After winning a scolarship to attend an independent Roman Catholic secondary school, he studied law at Aberystwyth University on the west coast of Wales and became a criminal lawyer in London.
He spent several years acting for hedge fund managers.
From law to politics
After becoming involved with UKIP, making his debut speech at their 2010 conference, he was elected to the European Parliament in 2014 to represent northwest England as the third pick on UKIP's list in the region.
A rising star within the party, he often advocated UKIP's case on BBC television's flagship "Question Time" weekly political debate programme.
Woolfe was seen as a figure to reach more voters in northern England, where it has recently emerged as a challenger in areas traditionally dominated by the main opposition Labour Party.
Woolfe stood in the 2015 general election as UKIP's candidate in the Manchester suburb of Stockport. He came third with 13 percent of the vote.
When the party's charismatic figurehead Nigel Farage announced he was stepping down as UKIP leader after Britain voted to leave the European Union in June, Woolfe was the odds-on favourite to replace him.
But he submitted his nomination papers 17 minutes after the deadline, citing computer problems.
The party's governing body rejected his candidature — which his supporters alleged was a coup led by opponents.
It also emerged he had failed to declare a drink-driving conviction when he stood for the Manchester police and crime commissioner post in 2012.
Fellow MEP Diane James won the leadership contest on 16 September, but backed out on Tuesday, saying she did not have sufficient support among colleagues.
Woolfe announced his candidacy again, in which he said he had considered defecting to Theresa May's Conservatives, after the new prime minister committed to carrying out Brexit.
"I came to the conclusion that only a strong UKIP can guarantee Brexit is delivered in full," he said.