Celtic have condemned the behaviour of Boli Bolingoli after the defender travelled to Spain on an unsanctioned trip in breach of Covid-19 quarantine rules.
The Belgian defender's action could bring the Scottish football season to a halt after he came on as a late substitute in Celtic's 1-1 draw at Kilmarnock on Sunday.
Now the reigning Scottish Premiership champions have apologised for Bolingoli's actions, saying 'it is difficult to imagine a more irresponsible action in current circumstances.'
Bolingoli failed to tell Celtic officials he had visited a high-risk country. Anyone arriving back in the UK from Spain has been required to self-isolate for 14 days since July 26.
Celtic manager Neil Lennon was unaware of the player's movements when he brought Bolingoli off the bench late on at Rugby Park.
In a statement released on Tuesday morning, Celtic expressed regret for the player's 'stupidity'.
It read: '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.'
It came as a spokesman for the Scottish Government suggested this latest breach - in the wake of several Aberdeen players testing positive for Covid-19 - could see the Premiership season halted just two games in.
'The Scottish Government is aware of reports of a Celtic FC player having broken quarantine rules last week,' the spokesman said. '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.'
After belatedly learning of the trip, furious Celtic bosses hauled Bolingoli in for a heated meeting at Parkhead on Monday.
On Monday night the defender - who has since tested negative for the virus - 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.'
The club has now launched an internal probe.
Former Celtic striker Chris Sutton said there should be 'no way back' for Bolingoli after his ill-advised trip to Spain.
Sutton tweeted: 'There should be no way back for Bolingoli at Celtic now. If he knowingly went to Spain before the Kilmarnock game and broke quarantine rules he showed he really doesn't give a s*** about his teammates, the manager and the support… Celtic should get rid immediately.'