The Scottish Government could halt the football season after Celtic player Boli Bolingoli breached quarantine rules.
The defender travelled to Spain and failed to self-isolate upon return, before playing against Kilmarnock on Sunday.
Spain is currently on the Scottish Government’s quarantine list, meaning those who travel to Scotland from the country have to self-isolate for 14 days.
The government is holding talks with Celtic and football authorities “to establish the facts”, but warned a pause on the game will be considered if there was a risk to public health.
Meanwhile, Celtic have said they condemn and apologise for Bolingoli’s behaviour, adding they “find it beyond explanation”.
They confirmed they will take action through their own disciplinary procedures, having revealed earlier they were investigating the matter.
A statement said: “Celtic Football Club unreservedly condemns and apologises for the behaviour of the player Boli Bolingoli in travelling to Spain without informing the club and in failing to observe quarantine restrictions.
“It is difficult to imagine a more irresponsible action in current circumstances and we find it beyond explanation. The club will take immediate action through our own disciplinary procedures.
“Celtic are in touch with every SPFL club today, and with all relevant authorities, to apologise for the fact that one of our employees has created so much additional difficulty through his actions.
“All of our playing and backroom staff have been tested twice for Covid-19 since this incident and all, including the player in question, have tested negative. While this comes as a relief to all concerned, it in no way diminishes the seriousness or stupidity of the player’s actions.”
The player himself, who was a late substitute in the 1-1 draw at Rugby Park, said: “I have made a huge mistake. I want to apologise to my manager, my team mates, the supporters, everyone at Celtic and so many others for letting them down so badly.
“I am guilty of a major error of judgement. I know what I did was wrong and I know that I must now deal with the consequences.”
A Scottish Government spokesman said it was “aware of reports of a Celtic FC player having broken quarantine rules last week”.
He added: “We are currently in discussion with the club and football governing bodies to establish the facts.
“If confirmed as another serious incident within Scottish football, where protocols have been breached at the risk of wider public health, then the Scottish Government will have little choice but to consider whether a pause is now needed in the resumption of the game in Scotland.”
Bolingoli joined Celtic on a four-year deal from Rapid Vienna last summer but has struggled to force his way into the first team on a regular basis.
Justice secretary Humza Yousaf described the player’s behaviour as “utterly unacceptable”.
He said the Scottish Government was “left with little choice but to consider whether pause is now needed in resumption of the game”.
Yousaf added: “Operational matter for Border Force and Police Scotland, I support whatever enforcement action they deem necessary.”
The breach comes after two Aberdeen players tested positive for Covid-19 – and a further six having to self-isolate due to close contact – following a visit to a bar in the city last Saturday.
The First Minister said the group “blatantly broke the rules” agreed by the Scottish FA, SPFL and Scottish Government.
Sam Cosgrove, Scott McKenna, Craig Bryson, Jonny Hayes, Bruce Anderson, Dylan McGeouch, Michael Devlin and Matty Kennedy, said they made a “huge error of judgement” in a statement.