The governing body of world gymnastics has demanded additional documentary evidence to prove that five gold medal-winning China gymnasts were old enough to compete in the Olympic Games.

The Chinese Gymnastics Association has been asked to supply further evidence to prove the birthdates of Jiang Yuyuan, Li Shanshan, Deng Linlin, Yang Yilin and He Kexin, after more questions emerged over the stated ages of He and Yang.

The five competitors won gold medals in the team gymnastics event, while He became the darling of the host nation after she won gold on the uneven bars and Yang picked up two bronze medals in individual events.

A US computer expert told The Times on Thursday that he had uncovered Chinese government documents proving that He and Yang were only 14. Both appeared to have been registered as two years younger in previous years.

Multimedia

• Simon Barnes in Beijing

• Bring back the tug-of-war

• Top 100 Olympic athletes of all time

Back to reality for golden generation

Olympic heroes will discover if the last 17 days has been a life-changing experience or a glorious but brief interlude

• Team GB to be rewarded with gold for gold

• Hard work starts now as British sailors look towards London 2012

• Public support gained from success in Beijing needs to last four years

BACKGROUND

• How the Brits became world beaters

• London double-decker arrives in Beijing

• Olympics proves capacity for greatness

• New dawn for British sport as Olympic sun sets

Related Links

• Commentary: why so upset? For the US little matters more

• IOC inquiry into 'underage' gymnast

• Hacker 'proves' gymnast underage

The online documents were the latest pieces of evidence to have emerged in recent months suggesting that the two athletes were two years beneath the minimum age of 16, in a sport where younger gymnasts are thought to have an advantage, being more flexible and thus better able to perform more difficult routines.

In a statement issued last night, the International Gymnastics Federation said that it was demanding additional evidence from the Chinese association “to resolve this issue once and for all”.

The federation did not specify precisely what documents it would require to satisfy itself that the two gymnasts were eligible. However, Lu Shanzan, China's coach, said that the documents that had now been passed to the federation included He's present and former passport, her ID card and family residence permit.

Lu said that all of these documents showed her to have been born in 1992 and complained that coaching staff and the girl's parents were indignant and upset at the investigation. “Surely it's not possible that these documents are still not sufficient proof of her birthdate,” he said. “The passports were issued by the Chinese Foreign Ministry. The identity card was issued by China's Ministry of Public Security. If these valid documents are not enough to clarify this problem, then what will you believe?”

The investigation has not been reported in the state newspapers. Leading Chinese news websites also failed to cover the story, and yesterday sites running reports of the inquiry appeared to have been closed down. Even the debates of the issue in internet chatrooms, which raged yesterday morning, disappeared in the afternoon as site-hosting companies moved to censor what could be a sensitive topic.

Some web commentators blamed the Americans for the inquiry and called for a retaliatory investigation of Michael Phelps, the US swimmer, who won eight gold medals at the Water Cube. Others were not surprised by the controversy. One wrote: “They will certainly now say that reports that she was 13 in 2007 were incorrect - so that she keeps her gold medal” - an apparent reference to a theory that He might previously have registered as younger than she was, in order to compete in a Chinese competition.

“Plank of Wood” commented: “Changing one's age in China, especially in the past, is very common. When I was at school I changed my age to get into the class I wanted. Later I changed it back.” All of these comments had vanished from the web by the afternoon.

The latter point was repeated by a former Chinese sportsman yesterday. He told The Times that in his youth he had once changed his age to participate in competition with younger players. “It used to be very common, but it is getting less and less so,” he said.

How would you strip athlete of a medal?

Under statute of limitations rules, the IOC and other sports bodies can go back eight years to request the return of medals and nullify competition results. This means that even if the He Kexin case becomes a protracted affair, as is likely, the Committee has until 2016 to decide if it wishes to act on the findings of the investigation by the Fédération Internationale de Gymnastique.

However, once a decision has been reached, action is usually swift. When last year Marion Jones, the American five-time 2000 Olympic champion, admitted using steroids, she was officially stripped of her medals within weeks.

Given that the IOC is based in Lausanne, on the banks of Lake Geneva in Switzerland, the usual method of return once the Games have ended is by postal courier. Two athletes have handed their medals straight back in Beijing - Ara Abrahamian, the bronze-medal winning Greco-Roman wrestler from Sweden, and Jong Su Kim, the North Korean shooter who won bronze and silver.

After disqualification, standings are normally readjusted, with the second-place finisher moving up to gold, third to silver and fourth to bronze.