More than 100 leading scientists say the Rio Olympics should be moved or postponed over the Zika outbreak.
The group says new findings about the virus make it “unethical” for the games to go ahead in an open letter to the World Health Organization.
They call on the WHO to urgently revise its guidelines on Zika, which is linked to serious birth defects.
The International Olympic Committee said it May it sees no reason to delay or move the games due to Zika.
The outbreak of the mosquito-borne disease began in Brazil a year ago, but now more than 60 countries and territories have continuing transmission.
Zika’s symptoms are mild, but in the letter the experts say it causes babies to be born with abnormally small heads and may also cause a rare and sometimes fatal neurological syndrome in adults.
The letter is signed by 125 international scientists, doctors and medical ethicists from such institutions as Oxford University and Harvard and Yale universities in the United States.
They cite the the failure of a mosquito-eradication programme in Brazil, and the country’s “weakened” health system as reasons to postpone or move the Olympics in “the name of public health”.
“An unnecessary risk is posed when 500,000 foreign tourists from all countries attend the Games, potentially acquire that strain, and return home to places where it can become endemic,” the letter says.
The biggest risk, it adds, is if athletes contracted the virus and returned home to poor countries that had not yet suffered a Zika outbreak.
The Rio Olympics take place between 5-21 August.
The WHO, which has declared the Zika virus a global public health emergency, is yet to comment on the letter.
On Thursday, the head of the US Centers for Disease Control and Prevention, whose advice is quoted approvingly in the letter, said the threat did not warrant halting the games.
“There is no public health reason to cancel or delay the Olympics,” Dr Tom Frieden said.
But he also urged the US to act more quickly to prevent pregnant women contracting Zika, amid congressional deadlock over the release of $1.9bn (£1.3bn) in funding.