The International Olympic Committee has asked South Korea to bar a soccer player from Saturday's medal ceremony after he was photographed apparently holding a political message about a territorial dispute with Japan following their bronze play off.
The player, who held up the sign after his team's 2-0 victory over Asian rivals Japan on Friday, risked inflaming an already tense political situation between the countries.
They have long argued over the islands which are known as Takeshima in Japan and Dokdo in South Korea, and which lie equidistant from the two nations.
On Saturday, Japan said it would take the territorial dispute to the International Court of Justice, after South Korean President Lee Myung-bak made a surprise visit to the islands this week.
After the visit, Japan recalled its ambassador to South Korea.
"We have opened an inquiry and have asked the NOC (National Olympic Committee) of the Republic of Korea for an explanation," the International Olympic Committee said in a statement.
"We have also requested that the Republic of Korea NOC takes swift action on this issue and that the athlete not be present at this afternoon's medal presentation ceremony."
The photograph seemed to show the midfielder Park Jong-woo, 23, holding the sign which read "Dokdo is our territory".
It swiftly spread on Twitter after being published by a Korean news agency.
"We just heard about what happened yesterday, we are looking into this incident but we are taking it very seriously," South Korean NOC chief of staff John Moon told Reuters.
"According to the IOC rules politics has no place in sport."
An official for the Japanese NOC said he was unaware of the incident.
It is believed the sign was shown long after the game had finished, in which South Korea won their first Olympic soccer medal.
Played at the Millennium Stadium in Cardiff in front of 56,000 sports fans, it had been a fiercely contested affair during which seven yellow cards were shown, four for the Koreans and three for Japan.
The disputed islands are believed to contain frozen natural gas deposits potentially worth billions of dollars.
Lee became the first South Korean leader to make the trip to the islands, which have been a persistent irritant in relations between the two countries.
Officials in South Korea said Friday's visit was meant to highlight the islands' importance as a natural reserve and was not aimed at stirring up trouble.
(Additional reporting by Choonsik Yoo and Karolos Grohmann, editing by Matt Falloon)