Beijing's Olympic plans have suffered an embarrassing blow over fears of the effect of pollution.
Australia's track and field athletes will be training in Hong Kong and have been told not to travel to Beijing until near the time of their events during the second week of the Games.
British athletes, who will train in Macau, have made the same decision but Australian authorities specifically cited Beijing's notorious pollution as a factor.
"We have had athletes come back from a recent test event and one athlete has got 10 days off training because of a respiratory problem," Max Binnington, the team's performance director, told Autralian radio. "We don't want our athletes to be undertaking that sort of risk."
Australia has led the way in demonstrating its concern over air quality in the Chinese capital, one of the most polluted in the world. John Coates, head of its Olympic committee, has also suggested that his competitors are at risk of food poisoning and infectious diseases, and has ordered an intense programme of vaccination.
Beijing has announced dramatic measures to improve its air, including banning half the capital's cars from the streets during the Games on alternative days, according to their licence plate, and shutting down polluting industry.
However, some of these plans have been scaled down from their original proposals, for fear of damaging economic growth.
The British Olympic Association said it was recommending that competitors should not take part in the opening ceremony if it is less than three days before their first event. Track and field athletes will also be staying in Macau until three days before the athletics events start.
However, it insisted this was to do with training routines and was the policy in place at previous Games, including Athens. Its medical advisers also say they are more worried about Beijing's summer heat and humidity than environmental conditions.
David Culbert, Athletics Australia's spokesman, said pollution was just one of the factors considered.
"It's a combination of factors, including the training facilities, whether the [Olympic] village is the best place to be or whether you're better off being outside the village," he said. "The heat, the humidity, the air quality, access to the training facilities, all of those things are a factor."
The Beijing Olympic Committee said it would be making no statement in response. But it is likely to be upset by the public nature of Australia's decision.
It comes on top of a series of escalating rows about arrangements for the Games. Broadcasters are complaining their equipment has not been given customs clearance to come into China, and are also demanding the right to broadcast live from around the city, including Tiananmen Square, without having to submit detailed applications in advance.
In an attempt to save Beijing's face, Mr Binnington insisted he was not intending to criticise China. "We think they will put on a wonderful show and they will do anything to minimise the inconvenience for athletes," he said.