An emergency World Health Organisation committee is meeting on Monday to advise on the response to the Zika virus, as the number of infected people continues to soar.

The committee will decide whether to designate the mosquito-borne virus – which has been linked to serious birth defects – a global emergency meriting immediate coordinated international action, amid criticism that it has been too slow to act.

On Thursday, WHO’s director general, Margaret Chan, said the virus was spreading explosively. The latest figures from Colombia, released over the weekend, backed her analysis, showing 20,297 confirmed cases of the disease in the South American country, including 2,116 pregnant women, making it the second most affected country after Brazil.

According to Colombian officials, the number of pregnant women confirmed to be affected has doubled in a week.

In Brazil, Zika has been linked, although not definitively so, to 4,000 cases of microcephaly, in which babies are born with smaller than normal heads, raising global fears about a virus that was previously considered relatively benign.

Revellers march at pre-carnival celebrations in Recife, Brazil. Health officials believe as many as 100,000 people have been exposed to the Zika virus in the city. Photograph: Mario Tama/Getty

Concerns have been amplified by the fact that the Olympics take place in Rio de Janeiro this summer, when about half a million tourists are expected to visit the city.

The WHO, which says there could be as many as 4m clinical cases of Zika in the Americas – although some experts think the figure could be much higher – is under pressure to come up with a speedy and effective plan of action.

The scientist who co-discovered Ebola has urged WHO to learn from its mistakes on its handling of that virus by quickly declaring a global health emergency over Zika.

Prof Peter Piot, director of the London School of Hygiene and Tropical Medicine, said: “By any means this [Zika] is a public health emergency – with the sheer number of people coming down with a flu-like syndrome”. The fact that some people developed neurological symptoms and the impact on the foetus were also a concern.

Speaking to BBC Radio 4’s Today programme, Piot added: “WHO clearly dropped the ball when it came to responding to the Ebola virus. It took about five months to declare the Ebola virus in west Africa as a public health emergency of international concern. Today that [WHO] committee is meeting and I have all confidence that they will declare this as a public health emergency. Several reports have been published [on the response to Ebola] and the WHO can, and must, do better.”

Piot said there was little danger of Zika spreading to the UK. “Zika and Ebola are different. The mode of transmission is different, that’s why I’m not concerned for the UK. There is no way that the aedes mosquito can spread here, it is too cold.”

Lawrence Gostin, a professor of public health law at Georgetown University in Washington, US, who has worked with WHO and written extensively about pandemics and policy, said Zika should have been declared an emergency as soon as the link between the virus and microcephaly was made. “My chief criticism is of WHO in Geneva,” he told Reuters. “After being widely condemned for acting late on Ebola, it is now sitting back with Zika.”

But officials at WHO, the United Nations health organisation, say the precise nature of any link between Zika and microcephaly remains unclear.

Although the disease does not pose the same threat to life as Ebola, and there has only been one case of possible person-to-person sexual transmission, according to the WHO, experts have warned that Zika poses a serious threat because of a number of its characteristics. These include the fact that an estimated 80% of infected people do not have any symptoms and that there is no vaccine in prospect.

The Aedes aegypti mosquitoes that transmit the virus thrive in urban conditions and are spreading as global warming takes effect.

An Indonesian research institute said on Sunday that it had found one confirmed Zika case on Sumatra island in a 27-year-old man who had never travelled overseas. The virus has previously been detected in a small number of people in south-east Asia, including one case in Indonesia, reports suggest. The Eijkman Institute for Molecular Biology in Jakarta, said it concluded that the virus had been circulating in the country “for a while”.