The Amazing Spider-Man actor reportedly attacked a friend of theatre producer Adam Spreadbury-Maher after inviting their group into his suite at the St Pancras Renaissance hotel early on Saturday morning (February 11).
The incident, which allegedly occurred around 3am, was witnessed by Ifans's girlfriend Anna Friel as well as actor Dominic Cooper, who had been partying with Spreadbury-Maher.
"It was a massive, beautiful suite... there were between 30 and 50 people there," the theatre boss told the Daily Mail. "I did not know who Rhys was but my friends told me he was a film star and host of the party.
"They invited us all in and were really lovely and then Rhys went mental and started on my friend. He was slapping him round the face and shoving him.
"I think he was trying to make himself feel like a big man. He went nuts. It was as though he was in a bubble and he didn't realise people were watching... I was livid... I think it's unreasonable someone can behave like that."
A spokesperson for the Metropolitan Police confirmed that they received a call about the alleged altercation at 6:24am.
"There was no report of serious injury," they said. "Officers attended the hotel at 10.46am and spoke with staff. Despite several attempts, we are yet to speak again with the informant or alleged victim and the investigation will remain open until we do."
The hotel's representative added: "An allegation of assault at the hotel was made in the early hours of Saturday. The police investigated and no evidence was found to support it."
Ifans and his team have yet to comment.
The 44-year-old was arrested for misdemeanor battery at last year's Comic-Con after allegedly pushing a female security guard. He was later released without charge.
Watch Rhys Ifans in the trailer for The Amazing Spider-Man below: