Wales have decided against challenging the £5,000 fine and four-week ban given to utility back Gareth Thomas.
Thomas is held back after clashing with Ulster fans in Toulouse
That means he is definitely out until 28 February, ruling him out of the Ireland, Scotland and France games.
The Toulouse back, 32, was found guilty of "aggressively attempting to enter one of the seating areas" during last month's Heineken Cup win over Ulster.
He was also found guilty by an ERC disciplinary panel of making an offensive hand gesture towards fans.
The former Wales and Lions captain, who received character references from former Labour party spin doctor Alistair Campbell and Matt Dawson, was found not guilty of "engaging in aggressive exchanges with spectators".
Whilst we think there are grounds for challenging the sentence, neither Gareth nor the WRU consider it would be in his best interests, or the best interests of the game, for an appeal to be lodged
The ERC panel was chaired by Jeff Blackett, the Rugby Football Union's disciplinary chief, and also comprising of Rod McKenzie (Scotland) and Achille Reali (Italy).
"We have carefully considered with Gareth Thomas the decision of the disciplinary committee and have also consulted with our lawyers," said Welsh Rugby Union group chief executive Roger Lewis.
"We have paid particular attention to the reasons given by the committee by way of justification of a sentence which we regard as severe.
"Whilst we think there are grounds for challenging the sentence, neither Gareth nor the WRU consider it would be in his best interests, or the best interests of the game, for an appeal to be lodged."
I will be standing shoulder to shoulder with the team throughout the Six Nations
Thomas himself added: "I would like to thank my team-mates, coaches and the WRU for the support I have been given during this very difficult time.
"Now I just want to resume training and support the Wales team to a victory over Ireland in Cardiff on Sunday.
"I will be with them heart and soul just like every other Welsh fan. I will be standing shoulder to shoulder with the team throughout the Six Nations."
An ERC statement said the starting point for the offence was a 12-week ban, but the sentence was reduced on the basis of Thomas' remorse.
"It is important that our sanction sends a clear message to the game at large that this sort of behaviour is entirely unacceptable," said the ERC.
This sort of behaviour is entirely unacceptable
ERC disciplinary panel statement
Thomas, who was set to start in Sunday's Six Nations opener with Ireland, had to be restrained from clashing with Ulster fans following an incident involving Toulouse team-mate Trevor Brennan on 21 January.
Former Ireland forward Brennan was due to face the same disciplinary hearing as Thomas in Dublin on Thursday, but he won a 24-hour injunction.
His lawyers argued the outcome of the hearing at the city's High Court could prejudice any criminal proceedings in France and affect Brennan's right to silence.
The ERC announced on Friday that Brennan's hearing will go ahead on 9 February, but only on whether or not it would prejudice a case in France.
Toulouse player Trevor Brennan lashed out at an Ulster fan
Brennan, 33, is accused of assaulting Ulster fan Patrick Bamford, leaving the 25-year-old accountant requiring medical treatment at the ground and on his return home to London.
Toulouse launched a robust defence of Brennan, describing the 33-year-old as a "generous man who has always respected opponents and their supporters".
The French club added: "It required sustained and repeated provocation to make him react in an unacceptable but understandably human way.
"We want European Rugby Cup (ERC) to investigate the behaviour of some of the Ulster supporters."
Ulster have already lodged a complaint with the ERC about the incident.
A statement said that that Brennan was involved in a "disgraceful incident and physically assaulted a member of the Ulster travelling support, Patrick Bamford".
The ERC have also announced further investigation into the alleged behaviour of a section of Ulster supporters, and Toulouse's alleged failure to control their players with regard to the incident.
Ulster have submitted a report of the incident to ERC after receiving a large number of written statements and photographs from eye-witnesses.