The marathon world record-holder said yesterday that he was unlikely to compete at the Beijing Olympics because China’s air pollution would damage his health.

The Ethiopian Haile Gebrselassie, 34, is a favourite for a gold medal this summer but is likely to focus instead on the 10,000 metres because he suffers from asthma.

“The pollution in China is a threat to my health and it would be difficult for me to run 42km in my current condition,” he said. “But I am not pulling out of the Olympics altogether.”

Jos Hermens, his agent, added that Gebrselassie, who has lung problems and a pollen allergy, did not want to damage his chances in the marathon at the London Games in 2012 and would confirm his decision next month once he was assured of a place in the 10,000 metres.

Dube Jillo, the technical director of the Ethiopian Athletic Federation, said: “Whether he runs in the marathon or 10,000 metres would be his own choice.”

Gebrselassie’s withdrawal would heighten concerns that Beijing’s notorious smog will not be cleared in time for the Games in August, when the weather in the city is at its most hot and humid.

A quarter of Britain’s top athletes suffer from exercise-induced asthma and some might wear face masks during training and competition. They include Paula Radcliffe, the marathon runner who succumbed to the heat before the finishing line in Athens four years ago. The mother of one has enlisted pollution experts in her campaign for her first Olympic medal.

The International Olympic Committee has said that it will postpone or cancel endurance events if conditions pose a danger to athletes’ health.

Gebrselassie, whose marathon record is 2hr 4min 26sec, called on China to deal with the pollution problem, which he feels will be “a hazard to athletes, seriously affecting their performances”.

Up to and during the Games, the IOC has promised to monitor Beijing’s air quality closely. On Sunday, Jacques Rogge, the IOC president, said that China had made “major progress” in tackling pollution. This included switching from coal to gas energy, closing 10 per cent of its power stations and planting millions of trees to improve its air quality.

Last week, the Chinese authorities began their first census of pollution sources around Beijing as part of an environmental improvement strategy ahead of the Games. The survey will cover 90,000 sources, of which 24,000 are industrial, 45,000 residential, 21,000 agricultural and 128 treatment facilities, according to Beijing’s bureau of environmental protection. Companies failing to provide accurate information face a fine of up to 50,000 yuan (£3,500). The findings of the survey will be approved in the first half of next year.

City officials said last month that significant pollutants had been reduced and air quality in the city had improved for a ninth consecutive year but experts have said that inhalable particles and ozone are still likely to be big issues during the Games. China has the top 16 most polluted cities in the world and 750,000 people a year die from breathing-related illnesses, according to a report published at the weekend.

The report from Human Rights without Frontiers International urged the IOC and Western governments to press China on the issue as well as on its human rights record. “China’s toxic air reflects its equally foul human rights record,” said Reggie Littlejohn, a US lawyer advising the Brussels-based group.

“It appears doubtful that Beijing will take the robust and decisive measures required to ensure safe air for the Olympics.”