Norwich's Olympic hopeful Nick Dempsey is concerned his bid for gold at the Rio 2016 Games will be undone by a plastic bag floating in polluted waters.
Britain's sailors have spent a lot of time training in Guanabara Bay, trying to predict the climate patterns, tides and currents to aid their bid for glory next month.
Yet Dempsey, who has been to Rio four times this year for two-week stretches, reckons the water quality is deteriorating and that someone's medal chances in the RS:X men's windsurfer class in which he competes will be ended by debris in the water.
Dempsey, who claimed silver at the London 2012 Olympics, said: 'Someone will lose a medal because of hitting something in the water.
'It's hitting the plastic bags in the water that's the problem. You stop, that's 10 places gone and you're not getting that back.
'It's going to happen – it's just how many times it happens to you. It's just going to be luck as to who gets the most plastic bags, hits a crate and that sort of stuff.'
Polluted waters can cause illness, too, and Dempsey, who returned from his most recent trip last Thursday, has not noticed an improvement despite the Games looming large.
He added: 'There's no improvements. It's worse. It was probably the worst it's ever been last week.
'It's just rubbish in the water, everywhere. It's a bit of a nightmare.
'There's nothing they (the Rio 2016 organisers) can do about it. It's a huge, massive infrastructure problem.'
Dempsey's training includes dealing with hitting debris, which is not difficult to incorporate when it happens so often.
'It happens every day in training, maybe three times that we're racing that we have to stop,' he added.
'It's become so part of it that we do practice stopping, getting something off quickly and going again.
'You have to completely stop, reverse, go sideways and it generally comes off straight away. It's quite a slow process.
'It's going to happen at a crucial moment. You've just got to get it off quickly.
'If it happens at the start line you will not finish top 15. That's your race done.'
Windsurfers are affected more by the debris due to the light weight of their vessel, while dinghy sailors can be soaked due to their low position on the water.
The British sailors are doing anything possible to combat the threat of illness and expressed regret more has not been done to clean up the bay.
Nick Thompson, world champion in the laser, said: 'When it rains a lot there's a lot of pollution in the water.
'It's a bit of a shame they haven't taken the opportunity to clean the waters up a little bit more.'
Hannah Mills, London 2012 silver medallist in the 470 with Saskia Clark, said: 'We're a bit sad that hasn't happened.'
British Sailing team manager Stephen Park hopes a final clean-up will still take place and says that contingency measures are available so the level of debris does not affect the racing unduly.
'Some of that rubbish tends to lie in particular areas because of the way the water moves,' Park said.
'If you've got to go across that line, as you approach that point you're looking closely in the water to see if you can avoid something.
'World Sailing are very conscious of that and if they think the rubbish is particularly bad, they'll probably move some racing to different courses.
'And, in theory, the Brazilians are going to do more to clean the courses.
'It is Brazil, they do have a habit of doing things very last minute. We'll wait and see.'
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