Tommy Robinson has said he was acting in "self-defence" when he punched another England fan to the ground ahead of the football match against the Netherlands.
Video footage shared on social media appears to show the EDL founder swinging at the man, who is sent sprawling to the ground outside the stadium in Portugal.
Robinson, 36, said he attended the Nations League semi-final in Guimaraes with his wife on Thursday night, and claimed he lashed out at a drunk England fan who had been "verbally threatening".
"I acted in self-defence," he said. "He was the aggressor."
Robinson said he threw the punch having already been approached twice by the man, who was with a group of friends.
"I had been trying to walk up there and he kept getting in my face," he said. "I thought he was going to punch me the first time."
Robinson said he was called a "w***r" and told: "F*** off. What you doing here?"
"It's self-defence because I'm not going to wait for him to beat me up in front of my missus," he added.
"He's come at me twice and I've said to his friends and him 'If you come at me again, I'm going to sort you out. Stay the f*** away from me'."
He said the incident happened in front of security staff, who allowed him into the game.
Robinson, who had two milkshakes thrown at him while out campaigning during his failed bid to become an MEP, added: "Any man out with their wife should be free to walk the streets without being threatened or abused because of their politics."
Deputy Chief Constable Mark Roberts, National Police Chiefs' Council's Football Policing Lead said: "We have been informed of an incident outside the Guimaraes stadium last night and the footage has been passed to our counterparts in Portugal for further investigation.
"As with all the disorder that has taken place in Portugal over the past few days, anyone found to be involved in disorder could be subject to a banning order on their return to the UK."
In 2011, Robinson was convicted of leading a street brawl involving 100 football fans and sentenced to a 12-month community rehabilitation order, 150 hours of unpaid work, and a three year football banning order.
He shouted "EDL till I die" and encouraged fellow Luton Town supporters as they clashed in the town with Newport County fans.