Finn Russell was told he would not be selected to face Ireland in Scotland's Six Nations opener after he failed to show up for Monday's training session following a late-night drinking session, it is understood.
The 27-year-old has been disciplined for a breach of team protocol following an incident at the team hotel last Sunday.
Russell's participation in the entire tournament is now in serious doubt after he turned down an offer to stay in camp and help Gregor Townsend's men prepare for next Saturday's Aviva Stadium clash.
The Racing 92 maverick flew back to Edinburgh from London on Sunday along with Saracens wing and national team colleague Sean Maitland after they had faced off in Heineken Champions Cup duty.
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But once reporting to the Dark Blues' team hotel, he headed straight to the bar.
When approached by team-mates and members of Townsend's back-room staff, he ignored requests to stop drinking and later left the hotel to return to his parents' home in Stirling.
Russell was a no-show at training the next day but did eventually report back to their Oriam base on the outskirts of the Scottish capital later on Monday evening. It was then he was informed he would not be selected for duty against the Irish.
During talks describe as "positive" by a Murrayfield source, Townsend did leave it open to the 49-cap stand-off to decide whether he wanted to stay in Edinburgh and help his team-mates get ready to face Andy Farrell's Irish outfit.
But Russell has opted to return to Paris and will not be among the travelling party who fly out for a warm-weather camp in Spain this Sunday.
The Scots host England in their second Six Nations fixture on 8 February, looking to retain the Calcutta Cup for a third successive year, and the Scotland coach has not closed the door on the former Glasgow star returning for that showdown with the Auld Enemy.
But as things stand it remains to be seen if Townsend's relationship with his talisman can be repaired.
The pair were famously embroiled in an angry half-time row at Twickenham last year as Russell protested against the tactics which had allowed England to race 31-0 ahead.
It was Russell who inspired the dramatic fightback which secured a thrilling 38-38 draw as he threw their original game plan out the window and went for broke.
Confirming the disciplinary action, the Scottish Rugby Union said in a tweet: "Stand-off Finn Russell will play no further part in preparations for Scotland's Six Nations opener against Ireland, having been disciplined for a breach of team protocol during the week's camp in Edinburgh. He has returned to his club."
The news will come as a major blow to Scotland fans as they look for improvement following a disappointing World Cup campaign, which started with a 27-3 defeat by Ireland and ended in elimination before the knockout stages.
The in-form Adam Hastings is now almost certain to wear the number 10 jersey in Dublin but Townsend may look for reinforcements with no other regular stand-offs in his squad, although full-back Stuart Hogg played there earlier in his career and Northampton centre Rory Hutchinson can also play in the position.
Glasgow's Pete Horne, who won his 46th cap in the Japan World Cup, was a notable omission from the original squad, while Duncan Weir is playing regularly for Worcester, having not represented his country since 2017.