The Scottish Government is to consider whether Scottish football should be "paused" - after another breach of the coronavirus protocols.
It has emerged that Celtic player Boli Bolingoli travelled to Spain, but did not quarantine on his return before playing in Celtic's 1-1 draw against Kilmarnock on Saturday.
The club say they were unaware of the situation until after the game.
It comes just days after eight Aberdeen players were forced to self-isolate, after going to a crowded bar in the city centre - where two of them caught coronavirus.
Justice Secretary Humza Yousaf says they now need to consider whether the Premiership campaign - which only got under way at the start of this month - should be allowed to continue:
Utterly unacceptable from Bolingoli. With a number of incidents, Scot Govt left with little choice but to consider whether pause is now needed in resumption of the game— Humza Yousaf (@HumzaYousaf) August 11, 2020
Operational matter for Border Force & Police Scot, I support whatever enforcement action they deem necessary https://t.co/fvIRsjhwZr
A Scottish government statement read: "The Scottish government is aware of reports of a Celtic FC player having broken quarantine rules last week.
"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."
Celtic said in a statement: "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.
"Since the Covid-19 crisis began, we have had a safe and healthy squad of players, recording not one positive test. As a club, we have worked tirelessly to ensure a sustained, safe environment for all.
"We have led the way in working with the football authorities and Scottish Government to establish the most rigorous, effective protocols and working practices, which ultimately led to the resumption of football. We could have done no more in this area.
"It is a matter of the deepest regret to us that the behaviour of one individual has jeopardised that reputation and reflected discredit on football more generally."
Reports suggest Bolingoli has apologised, saying: "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."