Colombian voters rejected a peace deal with Farc rebels in a surprise outcome that risks prolonging a 52-year-old conflict and plunges the country's future into uncertainty.

By a razor-thin margin of 50.25 to 49.75 per cent, Colombians voted against the peace accord, in a Brexit-style backlash that defied pollsters' predictions and left supporters of the deal in tears.

After nearly six years of negotiations, a half-century war that has killed 220,000 and displaced seven million from their homes is not over.

"I am the first to recognise the result," said Colombian President Juan Manuel Santos, flanked by members of the government peace negotiating team, who looked stunned. "Now we have to decide what path to take so that peace will be possible. ... I won't give up."


Surveys had predicted an easy win for the Yes vote by a margin of 2 to 1. Instead the result delivers a crushing blow to Santos, who since 2011 has pursued the peace deal with single-minded determination and to the steady detriment of his own popularity. He took an extraordinary risk by insisting that the accord - the product of tedious, grinding negotiations with the Farc - would only be valid if voters gave their blessing.

They didn't, and that failure has left Santos politically crippled. He told Colombians he would send his negotiating team back to Cuba today to meet Farc leaders. Santos also said he would meet Colombia's opposition, led by former President and senator lvaro Uribe, a mortal enemy of the Farc who has gained powerful new leverage over any possible attempt to rewrite the peace deal.

The outcome also amounts to a setback for the United States and the Obama Administration, which had backed Santos and pledged to boost US aid to Colombia by nearly 50 per cent, to US$450 million a year. The fate of that funding proposal is also now uncertain.

The vote was also an extraordinary rejection of the guerrilla commanders of the Farc, who in recent months have tried to engineer a makeover of the rebels' public image in preparation for an eventual return to politics. The outcome reveals the depths of Colombian public animosity towards the rebels, after decades of kidnappings, bombing and land seizures.

The vote, for many Colombians, was about far more than a ceasefire with Farc, or Revolutionary Armed Forces of Colombia. Many saw the country's political and judicial integrity at stake, and the peace accord as a dubious giveaway to the rebels.

What's next?

Opponents of the accord, led by powerful former President Alvaro Uribe, said if the accord failed to pass the Government and Farc should return to the negotiating table. Opponents specifically want to renegotiate provisions that would spare jail time for rebels who confess their crimes and give the Farc 10 seats in Congress until 2026. President Juan Manuel Santos says he will confer with Farc leaders. He also says he will consult with opponents of the accord. The Farc, which never wanted the referendum, has reiterated its commitment to finding peace, without saying if it will renegotiate. Both Santos and the Farc say the accord is the best one possible.

- Additional reporting: AP