Scrum-half Mike Phillips has been suspended indefinitely from Wales squad duties after a late night incident in Cardiff.
The Welsh Rugby Union (WRU) said he had been told not to report for training with the rest of the players on Monday.
It is seeking clarification about an incident outside a fast food restaurant in the early hours of Tuesday.
Police attended and said a man was taken away by officers but no arrests were made.
The WRU said while the player, aged 28, was on an agreed holiday period from pre-Rugby World Cup training it operated a strict code of conduct covering behaviour both on and off the field.
WRU group chief executive Roger Lewis said: "The player has been suspended due to a clear breach of the standards which we expect to be maintained by someone involved in one of our national squads.
"Discipline and acceptable behaviour are issues which run to the core values of the Welsh Rugby Union and we cannot tolerate any compromise of those standards.
"Mike Phillips is an exceptional player, but there is irrefutable evidence ... that he was engaged in behaviour which falls below the standards we set.
"It is vitally important that we send out a clear and unambiguous message to one and all concerning our views on matters of this sort.
"That message must be heard clearly by all players, coaches, administrators, volunteers, supporters and, of course, the parents and guardians of any child attracted to take part in the national sport of Wales.
"Our senior national squad players are highly visible because of the profile which accompanies the success of our national sport and it is made clear to them that they are the ambassadors we rely on to foster and sustain our image."
The incident occurred outside a McDonald's restaurant in Queen Street.
Footage taken by bystanders shows Phillips being restrained by police before being led away in handcuffs to a police van.
South Wales Police say he was not arrested.
The player was recently released by the Ospreys regional rugby team after securing a move to French club Bayonne.
The WRU said the suspension has been approved by Wales head coach Warren Gatland and agreed by the senior squad management team.
Wales team manager Alan Phillips said: "We have suspended Mike Phillips on the clear evidence of a level of behaviour we regard as unacceptable.
"It is clear there has been a breach of our disciplinary standards."
Cardiff Blues scrum-half Lloyd Williams has been called into the Wales Rugby World Cup training squad as a replacement and will report for duty on Monday.