Newly released footage of a sailor crashing into rubbish floating in Rio de Janeiro's Guanabara Bay has raised further concerns about the state's promises to clean up dirty waters where the Olympics sailing events will be disputed next year.

The incident, which took place on 14 February, involved professional sailors Breno Osthoff, 20, and Rafael de Almeida Sampaio, 35, who were training in a 49er class dinghy.

The video registers the moment when the boat, travelling at ten knots, approximately 20kmph, crashed into a plastic box used to transport fish which was discarded in the bay.

According to the Osthoff, the impact was so great that the box broke in half and the boat was forced onto its side.

"The impact broke the keel, damaged the hull, and as we turned we fell on top of the sails and scraped the two sails. The damage was very significant and it is just unacceptable," Osthoff said at a training session on 8 April.

There was costly damage to the boat, but Osthoff says that the potential damage to an athlete's sporting campaign could be much worse. He said that a collision of such kind, were it to take place during a competition, would be almost impossible to recover from. At Olympic level, he said, such a possibility cannot be permitted.

"Sailing in the rubbish is an atypical way to sail, since there are many conditions which we do not control but which we work with, such as wind conditions, temperature, direction and strength. We see a lot of rubbish but it is something we do not control. We often have to change our tactics due to unconventional factors," Osthoff explained.

Rio de Janeiro pledged to reduce pollution in the notoriously fetid Guanabara Bay by 80% but officials confirmed last month that the target will not be reached.

Brazilian Olympic officials said on 24 March that they would only clean up lanes for sailors in 2016 and not the whole body of water for the sailing competition as originally promised.

Rio Mayor, Eduardo Paes, said that the sailing events will take place in a relatively clean part of the bay and as it is the dry season, there will be less water flowing into the bay from the five rivers that surround it.

Biologists last year said rivers leading into the bay contained a superbacteria that is resistant to antibiotics and can cause urinary, gastrointestinal and pulmonary infections.