The Notting Hill star attended a pre-Bafta awards party in the city on Friday night (10Feb12) with his actress girlfriend Anna Friel, and later in the evening, he reportedly invited a number of people to a bash at his suite at the St Pancras Renaissance hotel.
Theatre producer Adam Spreadbury-Maher, 30, attended the party with friends including actor Dominic Cooper and he claims Ifans became embroiled in an argument with one of his pals, before slapping him in the face several times.
The alleged incident prompted Spreadbury-Maher to alert police.
He tells the Mail on Sunday, "There were a few of us, all actors and performers. That was around 3am. We were in a hotel room with Dominic, then there was knock at the door inviting us to another party... (It was) Rhys Ifans and Anna Friel's (suite) and there were between 30 and 50 people there.
"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. It was pretty terrible."
A spokesperson for the Metropolitan Police says, "I can confirm police were called at 6.24am on Saturday, February 11, by a man alleging that a friend had been assaulted earlier that night... There was no report of serious injury.
"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."
A spokesperson for Ifans says, "I can't get instructions (from my client)."