Four US Olympic cyclists who caused an outcry when they arrived at Beijing airport wearing smog masks have today apologised to Games organisers.

The four - Mike Friedman, Bobby Lee, Sarah Hammer and Jennie Reed - said that they were wearing the masks because of pollution fears, a touchy subject for the Chinese authorities.

As the Chinese capital remains shrouded in smog today, Jim Scherr, the chief executive of the US Olympic Committee, revealed that the four had said sorry.

"It probably wasn’t the most opportune time for these athletes to wear these masks,” he said, adding they had written an apology to Beijing Olympic Organising Committee (BOCOG).

But US officials said they would not ban US athletes from using masks to combat pollution if they felt it was needed because of air quality conditions.

“They have the right to wear masks if they feel it’s in their best interest to do so,” Scherr said.

“Hopefully they won’t have to use them and the air quality will be good."

Officials have today insisted once more that air quality is safe for athletes. Figures from China’s official Environmental Monitoring Centre deemed the air quality to be level two or “fairly good”.

Beijing enjoyed unusually blue skies last weekend following last-minute anti-pollution measures introduced on July 20, prompting optimism that organisers had managed to control air quality ahead of the Games.

Today, however, a murky haze hung over the main Olympic venues in northern Beijing, combining with high humidity and temperatures of 34C (95F) to create challenging conditions for competitors.

The quality of the Chinese capital’s air has proved a thorny public relations problem, but organisers said that plans to invoke further emergency measures before Friday’s opening ceremony would not be activated.

The authorities have already removed one million of the city’s 3.3 million cars from the roads and shut down more than 100 polluting factories and building sites in an attempt to clear the smog.

They said they were prepared to ban even more vehicles in Beijing and surrounding areas if conditions got worse, but the BOCOG spokesman said that organisers had backed away from such a move.

“The conditions are not unfavourable at the moment and my understanding is that we are not going to be taking any extra measures,” he told reporters.

Despite the reassurances from both Beijing organisers and the International Olympic Committee, some of the 10,000 visiting elite athletes continued to express their concern about the conditions.

The Spanish tennis player Fernando Gonzalez, who was playing a practice match with Wimbledon and French Open champion Rafael Nadal, said he was hoping for an improvement.

“The conditions are really tough with the pollution and it’s really hot. We don’t have too many days to adapt so I hope to be much better than today,” he said.

Jacques Rogge, the IOC chief, warned last year that poor air during the Games could result in the suspension of some events, particularly endurance races such as the marathon.

Ethiopian world record holder Haile Gebrselassie withdrew from the marathon because he feared the pollution could damage his health, although he will run the 10,000m.