LONDON (Reuters) - China has vehemently rejected suggestions of doping as a growing row over the astonishing performance of a Chinese swimmer threatens to overshadow Michael Phelps's bid to become the most decorated Olympian of all time on Tuesday.

Ye Shiwen, 16, is chasing a second gold in Tuesday's 200 meters individual medley final after winning the 400 medley on Saturday more than a second inside the world record.

"We want to be very careful about calling it doping," the American John Leonard, executive director of the World Swimming Coaches Association, told Britain's Guardian newspaper.

"The one thing I will say is that history in our sport will tell you that every time we see something - and I will put quotation marks around this - 'unbelievable', history shows us that it turns out later on there was doping involved."

But Xu Qi, head of the Chinese swimming team, told the news agency Xinhua: "Ye Shiwen has been seen as a genius since she was young, and her performance vindicates that ... If there are suspicions, then please lay them out using facts and data. Don't use your own suspicions to knock down others. This shows lack of respect for athletes and for Chinese swimming."

China briefly dominated women's swimming in the 1990s but their reign ended as fast as it began, following a series of doping scandals.

FASTER THAN LOCHTE

Ye swam the 400 medley five seconds inside her personal best, covering the last 50 meters of freestyle faster than American Ryan Lochte, who won the equivalent men's event in the second best time in history.

She issued a quick and firm denial on Monday, telling the China News Service: "My results come from hard work and training and I would never use any banned drugs."

Australia's Ian Thorpe, winner of five Olympic swimming golds, warned against rushing to judgment.

"Young swimmers can take off chunks of time that other swimmers can't," he said.

The chairman of the British Olympic Association, Colin Moynihan, said the World Anti-Doping Agency's (WADA) checks were extremely thorough. "She's been through Wada's program and she's clean," he said. "That's the end of the story. Ye Shiwen deserves recognition for her talent."

Xu said China "probably ranks number one in the world" for the frequency of its doping tests. "Our stance on doping is zero tolerance."

International Olympic Committee medical chief Arne Ljungqvist said it would ruin the "charm of sport" to raise doping suspicions every time an athlete's performance improved dramatically.

Others noted that American Phelps had broken his first world record at 15. "Michael Phelps is a phenomenal swimmer," British multiple short-course world champion Mark Foster said. "Is she the Chinese Michael Phelps? Why not?"

ALL-TIME RECORD

Phelps has gone on to win 17 Olympic medals, 14 of them gold. If he claims two more in Tuesday's 200m butterfly and 4x200m relay, he will overtake Soviet gymnast Larisa Latynina's record haul of 18.

He also has the chance in the butterfly, his favorite event, of becoming the first man to win the same swimming event in three successive Olympics.

"I made my first Olympic team in this. The shorter races are a lot better for me now that I'm older," Phelps said.

Monday provided another story of youthful success in the pool as Lithuanian swimmer Ruta Meilutyte, just 15, won the women's 100 breaststroke, and an upset as Frenchman Yannick Agnel beat Lochte in the men's 200 freestyle.

A total of three swimming golds put France third in the medal table at the end of Monday's third day of competition, behind the United States, on five golds, and China, on nine.

For the host nation, golds are proving elusive but a bronze in the men's team gymnastics on Monday felt almost as good as it ended a 100-year wait for any kind of a medal in the event.

The focus of home attention on Tuesday is Wimbledon as Andy Murray competes in the second round of the tennis competition, once again carrying the hopes of British fans yearning for a title after his final defeat by Roger Federer in the grand slam tournament there earlier this month.

Women's soccer throws up a tasty match between North Korea and the United States, at Manchester United's Old Trafford ground, that may prove as much of a spectacle for students of Cold War rivalry as for die-hard sports fans.