Water pollution concerns in Guanabara Bay have risen to the fore again ©Getty Images

A leading Rio 2016 official has given a "100 per cent guarantee" that all athletes competing at Olympic water venues will be safe, despite fresh concerns over rising pollution levels.

There have been claims that viral levels are 1.7 million higher than normal standards in the United States or European and that just three teaspoons of water could be enough to produce debilitating viruses.

Concerns over water quality have been among the biggest challenges for organisers since Rio de Janeiro were awarded the Olympic and Paralympic Games in 2009.

Much of the city's sewage still floats into Guanabara Bay close to the Marina de Gloria.

Dead animals and a severed human arm have been among physical items spotted.

Sailors have also been impeded by physical debrus during races.

Brazilian officials had aimed to reduce pollution levels by around 80 per cent but have fallen well short of this aim.

Even Rio Mayor Eduardo Paes has admitted they have failed to achieve their objective.

Other concerns also remain at other water venues, including in the Lagoa Rodrigo de Freitas for rowing and canoe sprint and Copacabana Beach for triathlon and open water swimming.

A total of 15 members of the United States rowing team were taken ill during last year's rowing test event.

The official response that this was unrelated to water quality was widely disputed.

International Olympic Committee (IOC) and Rio 2016 officials have repeatedly played down the scale of the problems.

IOC Coodination Commission chair Nawal El Moutawakel even promised she would jump into Guanabara Bay to prove it is clean.

She has not done so so far. 

Nawal El Moutawakel had vowed to jump into Guanabara Bay to prove how it is clean ©Getty Images
Nawal El Moutawakel had vowed to jump into Guanabara Bay to prove how it is clean ©Getty Images

Rio 2016 have also conducted only bacterial rather than viral testing.

This year, other issues such as security, Zika virus and the state of the venues have risen more to the fore but Associated Press have highlighted fresh concerns today.

They reported that 90 per cent of their test sites have shown high levels of viruses during 16 months of testing. 

They claim those who ingest only three teaspoons of water would have a high chance of contracting illnesses, including stomach and respiratory conditions  and, in a rarer case, heart and brain inflammation.

Tourists on Rio's beaches are also warned of dangers, with sand and beaches also showing high levels of bacterial markers.

"Don't put your head under water," Valerie Harwood, chair of the Department of Integrative Biology at the University of South Florida, told Associated Press.

Risks are thought to be particularly high for babies and small children.

RIo 2016 communications director Mario Andrada, however, has downplayed any risk.

"I can guarantee, 100 per cent, in the name of of Rio 2016 that athletes will be able to compete safely," he said here today. 

"We will do everything possible to ensure they will be safe."

World Sailing also claim to be encouraged by improvements after previously expressing concerns.

The governing body's chief executive Andy Hunt claims "considerable progress" has been made since March.