In the end there was no excuse, only an apology and acceptance, and with that Irish middleweight boxer Michael O’Reilly has written the most unwanted piece of Irish Olympic history by becoming the first Irish athlete to be sent home from the Games for failing a doping test.

It ends six days of uncertainty, weighed down by a heavy sense of inevitability, as O’Reilly accepted that the positive test, first reported last Thursday, “may have contained a prohibited substance” – while also claiming that he had “unintentionally” consumed it.

The first rule of any anti-doping code is strict liability, which means O’Reilly was always going to be responsible for the repercussions, the exact extent of which are yet unknown. He remains provisionally banned and with that his Olympics are over, three days before his scheduled opening fight on Friday at the Riocentro Pavilion.

The 23-year-old from Portlaoise Boxing Club was a medal favourite in his division, having won European Olympic gold in Baku last summer, only now he faces a potential ban of up to four years, even if he proves the supplement he consumed was unintentionally contaminated.

That’s a worry for another day, and will involve an anti-doping hearing on his return to Ireland. In the meantime, in a statement released through his legal representative, he also offered his “sincere apology to his fellow boxers, team mates, the Irish Athletic Boxing Association (IABA), Sport Ireland, the Olympic Council of Ireland and to all those who have supported him”.

According to his statement, he had “received notification on August 4th that an adverse finding had been made against him. The adverse finding arises out of a matter which was not deliberate or intentional.

“Having received further information, Mr O’Reilly now freely admits that he unintentionally took a supplement that may have contained a prohibited substance. Mr O’Reilly was given this supplement by someone unrelated to his team or association.”

This may clear the IABA and Sport Ireland from any direct involvement, although his claim that he “disclosed, at the time of the test, that he had taken the supplement” raises some further questions about both the source and the purpose of the supplement.

O’Reilly had previously indicated that rather than request the opening on the B-sample, he intended on presenting an appeal against the finding of the A-sample: however, given that strict liability rule, any such appeal was unlikely to be successful. He is also currently on the top state grant of €40,000 and that would be lost if he received a ban from the sport.

Following Steven Donnelly’s unanimous first-round win over Algeria’s Zonir Kedaches over the weekend, Irish coach Zaur Antia pleaded ignorance of the current status of his middleweight: “Zero tolerance towards doping, as always Ireland has a good resolution to fight against this. We don’t know what happened yet but we will see what happens later,” said the coach from Georgia.

Now, however, O’Reilly becomes the first Irish Olympic competitor to be sent home for a doping offence, although there was a previous case with an Irish horse.

During the Beijing Olympics in 2008, Denis Lynch was considered Ireland’s best chance of a medal, also in the individual show jumping, only for his horse, Lantinus, to be pulled on the morning of the final, again after a positive doping test. It later emerged Lantinus had also tested positive for a substance known to be on the banned list.

Four years before that, in the aftermath of Athens, Ireland was also forced to hand back an Olympic medal for the first time in our history: two months after Cian O’Connor had rode Waterford Crystal to the individual show jumping gold, it emerged his horse had tested positive for the banned substances fluphenazine and zuclopenthixol, both leg sedatives.

The investigation by the International Equestrian Federation decided O’Connor had not deliberately attempted to influence the performance of the horse, although he had to return the gold medal.

So far, there has been no official comment on the outcome from the IABA or the Olympic Council of Ireland (OCI). IABA president Pat Ryan arrived in Rio on Sunday originally intending on serving two roles: one as the head of the IABA along with chief executive Fergal Carruth; another as O’Reilly’s coach at the Portlaoise Boxing Club. Only now he only has to worry about the former.