The head of Britain's Olympic team today revealed athletes had been training in anti-smog masks to prepare for the Beijing Games.

Chef de mission Simon Clegg spoke at the flag-raising ceremony to welcome Team GB into the Olympic village.

Athletes woke this morning to a thick smog, temperatures of 37C and visibility down to a few hundred metres. The suffocating gloom descended on the Chinese capital after four days of clear skies.

City officials admitted pollution had worsened but said it posed no danger to competitors and emergency rescheduling of events would be unnecessary.

Mr Clegg, of the British Olympic Association, said the heat and humidity would be a factor but played down pollution levels. He said: "We've used the masks in training but we remain confident that these measures won't be necessary here [in China].

"People have been using them in their development since last year but no one is currently using them in Beijing." Mr Clegg declined to say which athletes used the masks, though 5,000m runner Mo Farah has been pictured wearing one.

One hockey player, part of the team that flew in from their acclimatisation base in Macau, told how players had been sprayed with fine ice and treated with iced towels to keep cool after training, and were having regular hydration tests. "They are doing their best for us but at the end of the day it is who can last out longest on the pitch," they said.

Poor conditions pose the greatest risk to endurance athletes such as Farah and Paula Radcliffe, who is asthmatic. She flew into Macau today.

Today in Beijing the "Bird's Nest" stadium was barely visible from the media centre a few hundred metres away and pedestrians were wearing face masks. Yesterday, the average air index was 35, according to officials. Below 50 is "excellent", 51 to 100 "fairly good", 101 to 200 "slightly polluted", 201 to 300 "poor", and over 301 "hazardous". The government has tried to tackle smog by moving factories out of town and restricting car access. Today Du Shaozhong, deputy director of Beijing municipal bureau of environmental protection, said: "By noon, the index is likely to be around 80."

However the official pollution level is consistently lower than that taken by the BBC.