Steven Woolfe, who on Wednesday announced his intention to lead the right-wing UKIP party, collapsed after the incident at the European Parliament in Strasbourg, France, according to interim party leader Nigel Farage.
Farage pledged to hold an investigation into the incident. "It's two grown men involved in an altercation," he told journalists in Strasbourg.
"It's not very seemly behavior. But I'm not today going to get involved in the blame game and name names. It shouldn't have happened."
UKIP said on its official Twitter feed that Woolfe had been "taken suddenly ill" and was undergoing tests in the hospital, but did not mention an altercation. Woolfe later released a statement saying he was "feeling brighter."
'Violence like this is abhorrent'
The circumstances of the incident that left Woolfe in hospital remain unclear. Raheem Kassam, a rival for the leadership of UKIP, issued a statement saying: "The details out of the EU parliament today are distressing. Our thoughts and prayers should be for Steven and his speedy recovery. Violence like this is abhorrent, and does not reflect UKIP or its members."
A spokesman for the Strasbourg police told CNN its officers had not been called to the parliament and that no investigation had yet been launched.
In a statement released from the hospital, Woolfe said he was feeling better. "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," he said. "As a precaution, I am being kept in overnight awaiting secondary tests to make sure everything is fine."
Riven by infighting
UKIP has been riven by infighting since Farage stood down from the leadership, the day after Britain voted by a narrow margin in a referendum to leave the European Union.
Woolfe had intended to stand in the original ballot to replace Farage, but failed to submit his nomination papers on time. The successful candidate, Diane James, stood down after 18 days
, saying she did not have the full support of the party's MEPs and office-holders.
Woolfe, 49, is the Member of the European Parliament (MEP) for North West England, having taken office in July 2014, and is the UKIP spokesman on migration and financial affairs.
He worked as a lawyer before entering politics, according to his website
There are 73 UK MEPs in total, of whom 22 are from UKIP.
Fellow UKIP MEP Bill Etheridge, who represents the West Midlands, tweeted his best wishes to Woolfe.