The `Notting Hill` actor - who is dating actress Anna Friel - is alleged to have slapped a guest in a late-night argument in a suite at London`s five-star St Pancras Renaissance hotel in the early hours of Sunday morning (12.02.12) after a pre-BAFTA Awards party.
Theatre producer Adam Spreadbury-Maher - who had been partying with a group of friends including Dominic Cooper at the Groucho club in London`s Soho - claims to have seen the 44-year-old star being verbally abusive to his friend before hitting him "quite a few times" in the face.
The 30-year-old creative director told the Mail on Sunday newspaper: "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. It was completely random. Everyone was having a lovely time. We were invited in and then he just went silly about 20 minutes after we arrived.
"He made a beeline for him and started shoving him saying, `What the f**k are you doing here? You`re a joker. Who the f**k are you?`
"He slapped him quite a few times. 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."
Earlier in the evening, Rhys had been to a dinner with Anna at an event hosted by `X-Files` actress Gillian Anderson and designer William Banks-Blaney at the St Pancras Renaissance hotel.
A Metropolitan Police spokesperson said: "I can confirm police were called at 6.24am on Saturday, February 11, by a man alleging that a friend had beenassaulted earlier that night at the Renaissance Hotel, Euston Road. 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."