Irish middleweight boxer Michael O'Reilly is understood to be preparing a challenge to the results of a drugs test threatening his participation in the Olympic Games, which get underway tonight.

Sources close to the boxer have told RTÉ News that he intends to fight a ban on competing in Rio after it was alleged he had tested positive for a banned substance.

The 23-year-old from the Portlaoise Boxing Club has been provisionally suspended from the games, which are due to officially start later today. 

He was tested before leaving for Brazil.

The middleweight fighter can request to have a 'B' sample tested, but under Irish anti-doping rules cannot participate in any competition or activity prior to the completion of an investigation. 

Meanwhile, captain of the Irish boxing team for the London 2012 Olympics Darren O’Neill, said he was very disappointed over the allegations and the effect it may have over team morale.

Speaking on RTÉ’s Morning Ireland, O'Neill said the news won't just affect boxers but will affect all of Team Ireland.

"It's just a disappointing outcome,” he said. "Not just for the boxers there but for the likes of myself, Kenneth Egan and Adam Nolan who have been doing this for many years.

"For Billy Walsh who built it up and to be honest even for the IABA President [Pat Ryan] who had been coaching Michael O'Reilly and would have been training Michael right up until the week they went away."

Also speaking on the programme, former President of the Irish Athletic Boxers Association Dominic O'Rourke said boxers are the most tested athletes in Ireland and are educated about drug taking.

O'Rourke said initially the news would have been a distraction for the other boxers, but they will quickly refocus.

"The team are professional enough, that they have a job to do out there," said O’Rourke. "They went out there focused for boxing and they would refocus again very quickly because it's a major task.

"These guys have been training for the past four years to get a shot at the Olympic Games, it's the chance of a lifetime for them and I don't think it will distract them that much."

The opening ceremony for the 31st Summer Olympics will be held in the Maracana soccer stadium in Rio de Janeiro tonight. 

Around 11,000 athletes will parade in the stadium in front of 50,000 spectators and a worldwide television audience estimated at around three billion.

Live coverage of the ceremony begins at 11.30pm on RTÉ 2.