CELTIC have launched an investigation into Boli Bolingoli after he travelled to Spain before featuring in the 1-1 draw against Kilmarnock on Sunday.
Manager Neil Lennon is said to be seething at the Belgian left-back for breaking UK and Scottish Government quarantine rules.
The champions were not made aware of the 25-year-old’s trip until after the game at Rugby Park where the player had come on near the end for Greg Taylor.
Bolingoli, who cost £3million when he arrived from Vienna Rapid last summer, went on holiday in secret and failed to self-isolate on his return.
Strict government rules dictate that anyone who visits Spain must spend two weeks in quarantine.
The Belgian went straight into training before travelling to Ayrshire with the first-team squad at the weekend.
It has been reported he was dragged into a club meeting yesterday to explain his actions and has since recorded two negative coronavirus tests.
Bolingoli, in a report in the Daily Record, 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 club statement read: “Celtic Football Club has taken its response to Covid-19 extremely seriously and we are pleased that, to date, we have recorded no positive tests.
“Our staff have given so much in this area, working tirelessly to ensure that all players and other club personnel are safe, fully aware of their own responsibilities and familiar with all guidance and protocols.
“Safety must always be our priority. Clearly, a full investigation will now take place and the club will take all appropriate action.
“Subsequent to the player’s return, he has recorded two negative tests in the past week.”
Bolingoli’s colleague Leigh Griffiths also had to be reminded of his responsibilities with regard to social distancing after he hosted a birthday party for his girlfriend.
That followed two Aberdeen players testing positive for coronavirus after a group of eight players from the club broke the rules by visiting a city centre venue.
That led to the Dons’ Premiership game at St Johnstone on Saturday having to be postponed.
The rearranged fixture will now take place a week on Thursday.