By Jack Stubbs and Karolos Grohmann
RIO DE JANEIRO (Reuters) - A global sporting tribunal opened the door wider to Russian participation at the Rio Olympic Games on Thursday, ruling that an Olympic ban on two Russian rowers and a swimmer with histories of doping was unenforceable.
The decision, on the eve of the opening ceremony, could invite a dozen more appeals against the ban from Russian competitors, according to the tribunal's chief, and underlines Russian arguments that the ban lacks legal justification.
"We can do an appeal in 24 hours, so anything is possible," Matthieu Reeb, secretary-general of the Court of Arbitration for Sport (CAS), told Reuters when asked if there was enough time for other athletes to have their ban overturned.
"It's a first step," he said, referring to the Thursday's rulings. "There's now potentially a way to get through."
In response to evidence of state-sponsored doping in Russia, the IOC last month directed sports federations to allow Russian athletes to compete only if they met certain criteria, including a clean doping past. Dozens of athletes were later excluded.
One of the athletes who could take advantage of the new ruling is whistleblower Yulia Stepanova who, together with her husband, ignited the Russian doping scandal by revealing the extent of the state-backed doping program.
Stepanova had been cleared to compete by the world athletics federation, which praised her courage in the affair, but the IOC refused to allow her to run in Rio due to her own doping past.
Russian officials in Rio could not be immediately reached for comment, but earlier the head of Russia's Olympic committee told reporters in the host city that the ban was discriminatory.
Alexander Zhukov said it was unfair Russian sports stars such as double Olympic champion pole vaulter Yelena Isinbayeva would now watch the Games from home while past doping offenders, including top U.S. runner Justin Gatlin, took to the field.
On Thursday, CAS ruled in favor of an appeal against the ban by four-times breaststroke world champion Yulia Efimova as well as rowers Anastasia Karabelshikova and Ivan Podshivalov.
Efimova had been disqualified from competing by swimming governing body FINA between October 2013 and February 2015 after testing positive for traces of the anabolic steroid DHEA.
CAS say the Olympic ban was unenforceable, saying an athlete cannot be sanctioned twice for a doping offense. FINA will now have to decide on her eligibility.
Karabelshikova and Podshivalov also argued that they should be allowed to compete given they had already served their doping suspensions. CAS sent their case back to the International Rowing Federation to decide on their eligibility "without delay".
Sport has been struggling for years to reconcile calls for a harder line on drug cheats against demands for natural justice.
The IOC had tried to ban cheats from the Games in 2011 with its own 'Osaka rule' - a suspension from the next Olympics for anyone with a six-month doping ban or longer - but that was also thrown out by CAS at the time.
A similar rule by the British Olympic Association to ban the nation's drugs cheats from Olympic participation for life was also rejected by CAS ahead of the 2012 London Games, allowing sprinter Dwain Chambers to race.
Several Olympic teams have competitors, who have been sanctioned for anti-doping rule violations in the past, in their ranks, including Gatlin and team mate LaShawn Merritt.
(Editing by Ossian Shine and Mark Bendeich)