A Brazilian group of lawyers, activists and scientists is to ask the country’s supreme court to allow abortions for women with the Zika virus.
Zika, a form of microcephaly, is linked to brain defects in unborn children.
Abortions are illegal in Brazil, except in health emergencies or cases of rape or, since 2012, another brain condition known as anencephaly.
Three to four million people could be infected with Zika in the Americas this year, experts have warned.
Meanwhile, Thomas Bach, the head of the International Olympic Committee, said steps were being taken to protect this year’s Games in Rio de Janeiro.
The IOC will issue guidelines later on Friday for athletes and visitors taking part in the games.
The new petition is to be delivered to the supreme court in two months’ time. The BBC has learned that it argues that “the Brazilian state is responsible for the Zika outbreak” for not having eradicated the Aedes aegypti mosquito which carries it.
Brazilian women “should not be penalised for the consequences of flawed policies”, it says.
The group behind the microcephaly supreme court plea also won the exception for anencephaly in 2012.
Brazil is the country worst affected by the Zika outbreak, with 270 cases of microcephaly confirmed by the health ministry and 3,448 being investigated.
Debora Diniz, a law professor at Brasilia University, told the BBC the disease disproportionately affected the poor.
“What we have at this moment is a group of women who are in fear of getting pregnant and do not know what will happen during the pregnancy,” she said.
She added: “It is important to remember, when we talk about abortion and reproductive rights in general, that we have a social class split in Brazil – wealthy women will access safe abortion, legal or illegal, and poor women will go to the illegal market or continue to be pregnant.”
Professor Diniz has called for better access to prenatal care and earlier screening for diseases such as microcephaly. She said: “This is not only an abortion case, this is a women’s rights case.”
Most people do not develop symptoms of the Zika virus but may pass the virus on to their children. There is no known cure or vaccine. The US says it hopes to begin human vaccine trials by the end of 2016.
Officials from the US National Institute of Health (NIH) said they had two potential Zika vaccines in development. One that is based on an experimental West Nile vaccine could be repurposed for Zika and enter clinical trials by the end of 2016, the NIH said.
WHO director general Dr Margaret Chan said Zika had gone “from a mild threat to one of alarming proportions”. She has set up a Zika “emergency team” following the “explosive” spread of the virus.
The team will meet on Monday to decide whether Zika should be treated as a global emergency. The last time an international emergency was declared was for the Ebola outbreak in West Africa, which has killed more than 11,000 people.
Zika was first detected in Uganda in 1947, but has never caused an outbreak on this scale. Brazil reported the first cases of Zika in South America in May 2015.
WHO officials said between 500,000 and 1.5 million people had been infected in Brazil, and the virus has since spread to more than 20 countries in the region.
What is the Zika virus
- Spread by the Aedes aegypti mosquito, which also carries dengue fever and yellow fever
- First discovered in Africa in the 1940s but is now spreading in Latin America
- Scientists say there is growing evidence of a link to microcephaly, that leads to babies being born with small heads
- Can lead to fever and a rash but most people show no symptoms, and there is no known cure
- Only way to fight Zika is to clear stagnant water where mosquitoes breed, and protect against mosquito bites