The Sun reports that the TV presenter was with friends when he was taunted by another drinker at the The Barley Mow in Chiswick, West London.
Giedrius Kudzinskas told the paper that the attacker had shouted, "Where's your girlfriend?" at McPartlin - claimed to be a reference to his co-host Declan Donnelly.
"Ant didn't react and this bloke launched himself at him and punched him in the face," the eyewitness continued. "Ant reeled back and then a big fight broke out as his mates came to help him. Other people jumped in and held back the other guy.
"The bloke was thrown out but when Ant left later, he was waiting for him. There was another lot of shouting and then Ant walked off. It was all totally unprovoked."
Claiming to have spoken to McPartlin following the incident, Kudzinskas added: "He was furious about what he had said to him."
Declan Donnelly and his girlfriend Georgie Thompson had been at the bar but left prior to the attack.
A spokesperson for the Push The Button star has apparently confirmed the attack but said that McPartlin would not be taking the matter further.