Update: 8:15PM The World Health Organisation (WHO) meets tomorrow morning to decide whether to classify the Zika outbreak as a global health emergency.

The WHO last week warned that the virus, which has been linked to serious birth defects in Brazil, was spreading rapidly, with up to 4 million cases expected in the Americas this year.

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WHO officials have described zika as moving “from a mild threat to one of alarming proportions”.

Most cases will have no symptoms but the virus has been linked to brain abnormalities in thousands of babies in South America.

Declaring a “public health emergency of international concern” would establish Zika as a serious global threat and lead to money, resources and scientific expertise being thrown at the problem both in South America and in laboratories around the world.

The WHO’s actions are under intense scrutiny after its handling of the outbreak of Ebola in West Africa.

Its efforts to prevent the spread of the virus were widely criticised and it was deemed to have been too slow to declare an emergency.

At the meeting, experts in disease control, virology and vaccine development will brief WHO director general Dr Margaret Chan.

Last week, she said: “The level of concern is high, as is the level of uncertainty.

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“For all these reasons, I have decided to convene an emergency committee.

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“I am asking the committee for advice on the appropriate level of international concern and for recommended measures that should be undertaken in affected countries and elsewhere.”

Since the mosquito-borne disease was first detected in Brazil in May 2015, the virus has spread to more than 20 countries.

The biggest concern is the surge in levels of microcephaly – babies born with abnormally small heads – and the rare nervous system disorder Guillain-Barre syndrome.

The link between the virus and these disorders has not been confirmed, but Dr Chan said it was “strongly suspected” and was “deeply alarming”.

And she warned the situation could yet deteriorate as “this year’s El Nino weather patterns are expected to increase mosquito populations greatly in many areas”.

An announcement on whether Zika constitutes a global emergency is not expected until at least Tuesday.