China has become embroiled in the first doping controversy of the London Games after one of the world's most respected coaches described the swimming prodigy Ye Shiwen's gold medal performance as "unbelievable" and "disturbing".

The American John Leonard, executive director of the World Swimming Coaches Association, said the 16-year-old's performance was "suspicious" and said it brought back "a lot of awful memories" of the Irish swimmer Michelle Smith's race in the same event at the Atlanta Olympics in 1996. Smith, now Michelle de Bruin, was banned for four years in 1998 for tampering with a urine sample.

Ye stunned world swimming on Saturday by winning gold in the 400m individual medley in a world-record time. It was her final 100m of freestyle, in which she recorded a split time of 58.68sec, that aroused Leonard's suspicion. Over the last 50m she was quicker than the American Ryan Lochte, who won the men's 400m individual medley in the second-fastest time in history .

"We want to be very careful about calling it doping," said Leonard, who is also the executive director of the USA Swimming Coaches Association.

"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. That last 100m was reminiscent of some old East German swimmers, for people who have been around a while. It was reminiscent of the 400m individual medley by a young Irish woman in Atlanta."

Leonard is the first major figure in the swimming world to go public with suspicions over Ye's performance. London 2012 organisers and the World Anti-Doping Agency (Wada) have insisted that anyone cheating at the Games would be caught, with a record 6,250 tests being carried out.

About half of the 10,500 athletes, including all medal winners, will be tested for 240 banned substances. But Wada has also repeatedly raised concerns about athletes who may be successfully doping out of competition, drawing a distinction between them and "dopey dopers" who are caught during a major championships.

Stephanie Rice, the Australian who won gold in both women's medley events in Beijing in 2008, described Ye's performance as "insanely fast". Ariana Kukors, the 2009 world 200m medley champion from the US, has said it was "amazing" and "unbelievable".

Leonard, who said Ye "looks like superwoman" added: "Any time someone has looked like superwoman in the history of our sport they have later been found guilty of doping."

His comments are liable to further increase tensions between China – which has poured huge resources into its sporting programmes in recent years and topped the US in the medal table for the first time in Beijing four years ago – and the Americans.

Ye was more than seven seconds faster in the Olympic 400m individual medley final than she had been in the World Championship equivalent last July.

Leonard said that although this vast improvement was possible, it would be very hard to achieve. "But the final 100m was impossible. Flat out. If all her split times had been faster I don't think anybody would be calling it into question, because she is a good swimmer. But to swim three other splits at the rate that she did, which was quite ordinary for elite competition, and then unleash a historic anomaly, it is just not right."

Asked about the accusation that she was doping, Ye replied: "The Chinese team keep very firmly to the anti-doping policies, so there is absolutely no problem."

Leonard also questioned why Ye was not competing in the 200m or 400m freestyle, despite her phenomenal performance in that discipline in the medley, saying that was one of "a whole bunch of other questions".

He has been executive director of the WSCA since 1989. "I have been around swimming for four-and-a-half decades now," he said. "If you have been around swimming you know when something has been done that just isn't right. I have heard commentators saying 'well she is 16, and at that age amazing things happen'. Well yes, but not that amazing. I am sorry."

Leonard said that the consensus in the coaching community he represents was that the swim was "unbelievable". "I use that word in its precise meaning. At this point it is not believable to many people," he added.

"No coach that I spoke to yesterday could ever recall seeing anything remotely like that in a world level competition," Leonard continued. "Where someone could out-split one of the fastest male swimmers in the world, and beat the woman ahead of her by three-and-a-half body lengths. All those things, I think, legitimately call that swim into question."

Ye also won the 200m medley at the World Championships in 2011, and qualified fastest for the semi-finals of that event in Monday morning's heats, in a time that was 1.61sec quicker than her nearest competitor.

Leonard also argued that it was fair to point to the positive tests incurred by Chinese athletes in the past. In 2009 five junior Chinese swimmers were banned after testing positive for the anabolic agent clenbuterol at the 2008 national junior championships .

"You can't turn around and call it racism to say the Chinese have a doping history," Leonard said. "That is just history. That's fact. Does that make us suspicious? Of course. You have to question any outrageous performance, and that is an outrageous performance, unprecedented in any way, shape or form in the history of our sport. It by itself, regardless of whether she was Chinese, Lithuanian, Kenyan, or anything else, is impossible. Sorry."

Leonard rejected comparisons to Michael Phelps, who broke the 200m butterfly world record when he was just 15, back in 2001 because the American got "consistently faster every year on a normal improvement curve".

He said he had no qualms about the performance of other Chinese swimmers, including the new Olympic 400m freestyle champion Sun Yang, 20. "He has a perfectly normal improvement curve, he is a dramatically spectacular athlete in our sport and I've no question about him at all. But a woman does not out-swim the fastest man in the world in the back quarter of a 400m IM that is otherwise quite ordinary. It just doesn't happen."

Blood samples taken at these Games will be kept for eight years. "I am sure that Fina and the doping authorities have taken every sample they can take," Leonard said. "The sample will be tested and available for testing for the next eight years. And over eight years, if there is something unusual going on in terms of genetic manipulation or something else, I would suspect over eight years' science will move fast enough to catch it. I have every faith that eventually if there is something there to be caught it will be caught. Right now all we can say is Olympic champion, world record holder, and watch out for history." But Arne Ljungqvist, chairman of the International Olympic Committee's medical commission and a veteran anti-doping official, said that as yet he had no particular suspicions around the Chinese swimmer.

International Olympic Committee medical commission chairman Arne Ljungqvist says he has 'no suspicions' over Ye Shiwen. ITN

"Should I have my suspicions I keep them for myself, first of all, and take any action, if so, in order to find out whether something is wrong or not. You ask me specifically about this particular swimming. I say no, I have not personally any reason other than to applaud what has happened, until I have further facts, if so."

Ljungqvist added that he was unaware of which athletes had been tested in the build-up to the Games. He described the IOC's mandate as "limited" because its testing programme only covers the period from when the athlete's village opens.

"We have a testing programme, as you know, that covers only the period from the opening of the village until the end of the Games, and any doping programme would probably be put in place long before then," he said. "So our mandate is pretty limited and it is therefore very much a matter of the international federations and the national Olympic committees to make sure that athletes are clean when they come here."

Ljungqvist said that sudden advances in performance could bring athletes under closer scrutiny but said "sport is in danger" if surprise performances automatically provoke suspicion.

"We are using many reasons for having target testing. Of course should a sudden rise in performance occur in a particular person, we could regard that possibly as a reason to do it, but I would rather say that it is tragic if that should be the primary reason for doing a testing."Calls to the Chinese Olympic Committee's listed press attache for the London Games, Zhang Haifeng, went unanswered on Monday.

With athletes willing to cheat caught in an ongoing arms race with anti-doping authorities employing ever more sophisticated means to catch them, they continue to be caught doping. On the eve of the London Games, the International Association of Athletics Federations banned nine athletes.

IOC president, Jacques Rogge, in his opening press conference, said the fact that doping cheats were being caught and banned was a positive sign and said the fact samples would be held for up to eight years was a major deterrent. Three athletes have been sent home for doping offences since the Games began.

"As far as the athletes being caught positive before the Games, this is a good sign for the fight against doping. In all, in total, 107 athletes were caught positive in the two months preceding the Olympic Games," he said.

"We are continuing to test and test and test again before the competition. We will be testing, of course, during the competition, but I will say that this is proof that the system works, that the system is effective and that the system is a deterrent one."

Jeremy Hunt, the culture secretary, said anyone doping at the Games would be caught. "China gets more gold medals than any other country so they're always going to be a target as the top dog. They are outstanding athletes, but we need to remember that this is the most heavily policed Olympics ever in terms of doping," he said. "The regime is incredibly thorough and incredibly strict. So if there are people who are doing what they shouldn't, we can be as confident as we'll ever be that they will be found out."

• This article was amended on 31 July 2012 because the original said Michelle Smith was banned after testing positive for an anabolic steroid. This has been corrected.