An association between the Zika virus and a congenital brain condition found in babies born to infected mothers should be considered a "public health emergency of international concern", the World Health Organisation has said.
The global health body made its decision after an emergency meeting in Geneva to discuss the "explosive" nature of the virus.
WHO officials have predicted as many as four million people could be infected with the virus this year.
The last time a global emergency was declared was for the Ebola outbreak, which is thought to have led to more than 11,000 deaths.
Zika has been linked to thousands of babies being born with underdeveloped brains in Brazil.
Colombia has also seen a rise in the number of patients diagnosed with a rare neurological disorder which can cause paralysis.
Following a meeting of an International Health Regulations Emergency Committee, WHO director general Dr Margaret Chan said the causal relationship between infection during pregnancy and microcephaly in babies is "strongly suspected" but not scientifically proven.
The committee advised that the association between the virus and microcephaly - a condition where the child has an underdeveloped brain - constitutes an "extraordinary event".
She said a co-ordinated international response was needed to investigate and understand the relationship between the virus and the condition.
"After a review of the evidence the committee advised that the clusters of microcephaly and other neurological complications constitute an extraordinary event and a public health threat to other parts of the world," she said.
"In their view a co-ordinated international response is needed to minimise the threat in effected countries and reduce the risk of further international spread.
"Members of the committee agree that the situation meets the conditions for a public health emergency of international concern.
"I have accepted this advice. I am now declaring that the recent cluster of microcephaly and other neurological abnormalities reported in Latin America, following a similar cluster in French Polynesia in 2014, constitutes a public health emergency of international concern."
Dr Chan added: "The committee found no public health justification for restrictions on travel or trade to prevent the threat of Zika virus.
"At present the most important protective measures are the control of mosquito populations and the prevention of mosquito bites in at-risk individuals - especially pregnant women."
Since the start of the outbreak last year, five UK travellers have been diagnosed with the Zika virus.
While many do not even know they are infected, some can suffer symptoms including fever, joint pain, itching, rash, conjunctivitis or red eyes, headache, muscle pain and eye pain.
In the UK, the National Travel Health Network and Centre recommends that people who are pregnant or trying to becoming pregnant should reconsider travel to affected countries.
It has advised that any patients who suffer from a severe, chronic medical condition, or have medical conditions that weaken the immune system, should seek advice from health workers before travelling.
It has also urged health professionals to consider Zika as a possible diagnosis in any patients with fever returning from South and Central America, the Caribbean, Africa, south and south-east Asia and the Pacific region.
Public Health England (PHE) said men in the UK should wear condoms for a month after returning from any of the 23 countries affected by Zika.
PHE said the risk of transmission of the virus through sex was very low but condoms should be used as a precaution.
At present, there are no vaccines, specific treatments or rapid diagnostic tests for the virus.
Dr Jeremy Farrar, director of the Wellcome Trust, said: "The WHO faced heavy criticism for waiting too long to declare the Ebola outbreak a public health emergency and they should be congratulated for being far more proactive this time."