They have battled dense smog, strong tides and no wind but now British sailors training for the Beijing Games are contending with mutant seaweed that has invaded the Olympic venue in China.

The bright green algae, described as “thick as a carpet”, is making it impossible for dinghies to navigate the course that will host the Olympic regattas in less than two months.

Wrapping itself around keels, bringing the boats to a standstill, the seaweed is believed by experts to have drifted in from the Yellow Sea to the eastern coastal city on the back of bad weather during the past month. Local fishermen are struggling to clear the area, armed only with their nets slung over the side of their small boats.

Iain Percy, a gold medal-winner in Sydney 2000, said that the ocean had “turned green”. He told The Times: “It has appeared in patches about the size of football pitches which are impossible to sail through. How the organisers are going to be able to set a mile-long sailing course is a real concern.”

The theories about its cause include recent inland flooding caused by typhoons and global warming. Chinese officials deny that its presence in Qingdao — now living up to the Chinese translation of “green island” — is the result of pollution.

“What seems to be the fishing fleet of eastern China has been moved on to seaweed collection duty,” said Mr Percy, who is competing in the doublehanded keelboat class.

Even the windsurfers are struggling. Bryony Shaw, a gold medal hopeful in the women's class, said: “The seaweed is as thick as a carpet, there's no way you can sail through it. If it's still here in August, it could be a real problem.”

Sailing coaches fear that the seaweed could even change the tactical dynamics of the Olympic events. Technically it could be classed as an obstruction, in which case sailors would have to manoeuvre around it.

Depending on its drift, it could give one boat the edge over a rival.

“If you're allowed to call it an obstruction, that actually changes the rules of sailing,” Miss Shaw said. “As the current is so strong, you can begin racing in one area clear of the weed, and then ten minutes later it's drifted into your sailing area.”

The seaweed is another blow to the Chinese organisers, who boasted recently of the technological advances they were introducing to the sailing venue to combat the meteorological challenges at Qingdao. These include China's first vehicle-borne “doppler” radar, used in police speed guns, which will provide information on wind speed and the direction of the tides.

Ben Ainslie, Britain's most successful Olympic sailor, has described the venue as “a sailor's nightmare”.

Sailing is one of Britain's strongest events, with nine medal hopes. A failure to produce a clutch of champions because of adverse conditions would dent Britain's chances of meeting its target of eighth in the medals table.

Analysis: Lewis Smith

Algal blooms are usually prompted by pollution, which provides the organisms with so much food that they multiply many times faster than usual.

Outbreaks in the ocean can sometimes be seen from space.

Seaweed, despite its superficial similarity to land plants, is a large form of algae that can bloom just as its microscopic cousins do.

Algal blooms can promote the rapid growth of bacteria that absorb so much oxygen that they create “dead zones” in which other wildlife is killed off.

It is thought that marine blooms are on the rise because of an increase in chemicals that get into coastal waters.

In the Baltic Sea overfishing is believed to have increased the frequency of blooms because the loss of cod allowed more algae to grow instead of being eaten.