By Peter Rutherford

SEOUL (Reuters) - The Court of Arbitration for Sport cleared the way for former Olympic swimming champion Park Tae-hwan to compete at the Rio Games on Friday after upholding his appeal against a controversial Korean Olympic Committee doping ban.

Park, the first Korean to win an Olympic swimming medal when he won gold in the 400 freestyle gold in Beijing, completed an 18-month ban imposed by world governing body FINA in March after testing positive for testosterone ahead of the 2014 Asian Games.

However, under a KOC regulation, he was then hit with an additional three-year ban from the national team the day the FINA suspension expired, effectively ruling him out of contention for the Rio Olympics.

Park took his case to CAS, sport's highest tribunal, and also filed an injunction against the KOC and Korea Swimming Federation with Seoul Eastern District Court, which ruled last week he should be considered eligible for selection.

On Friday, the deadline for South Korea to submit its list of Rio swimmers to FINA, CAS ruled that Park was eligible for selection for this summer's Games.

"The decision issued by the President of the CAS Appeals Arbitration Division means that he is eligible to be selected to swim for the Korean team in the Rio 2016 Olympic Games," CAS said in a news release.

'MODIFY SELECTION RULE'

Following the ruling, the KOC said they would abide by the decision.

"As one of the IOC's National Olympic Committees, the KOC respects the CAS decision and will take action to modify its rule on national athlete selection," it said in a statement.

The decision comes as a blow to the KOC, which had shown no sign of wavering from its hard-line anti-doping stance since Park's FINA ban ended last year.

However, CAS has previously struck down double-barrelled punishments for athletes banned for doping-related offences and Friday's ruling was widely expected.

In 2011, the Swiss-based tribunal ruled that the International Olympic Committee's 'Osaka Rule', which banned athletes serving suspensions of at least six months from competing at the next Games, violated its own statutes.

Despite the KOC ban, Park entered national swimming trials in April and won all four of his races in times quick enough for Olympic qualification.

The swimmer's father Park In-ho said his joy was tempered.

"I feel heavy rather than happy, because we took a long and bumpy road to get here. I am grateful to all the fans and people who encouraged Park Tae-hwan when he had the most difficult time."

The 26-year-old swimmer, who also has three silver medals from the Beijing and London Games, will have his work cut out to win a fifth Olympic medal in Rio as his best times of the year for the 200 and 400 freestyle are well off the world-leading marks.

(Additional reporting by Nataly Park and Lee Jeung Eun; Editing by Nick Mulvenney/Amlan Chakraborty)