Voters in a referendum in Colombia have rejected a landmark peace deal with Farc rebels in a shock referendum result, with 50.24% voting against it.
The deal was signed last week by President Juan Manuel Santos and Farc leader Timoleon Jimenez after nearly four years of negotiations.
But it needed to be ratified by Colombians in order to come into force.
Farc rebels agreed to lay down their weapons after 52 years of conflict in order to join the political process.
President Santos has previously warned that there is no plan B for ending the war, which has killed 260,000 people.
With votes in from more than 99% of polling stations counted, 50.2% opposed the accord while 49.8% supported it - a difference of less than 63,000 votes out of 13 million ballots.
The surprise result means the peace process is now shrouded by uncertainty.
It is also a major setback to President Juan Manuel Santos, who since his election in 2010 had pledged to end a conflict blamed for displacing about eight million people.
Less than a week ago, President Santos was celebrating with world leaders and Farc commanders the end of Latin America's last and longest-running armed conflict at a ceremony in the historic city of Cartagena.
The rebels were making plans to lay down their weapons and become a political party within six months.
But the president is now facing one of the most difficult moments in Colombia's recent history, says the BBC's Americas Editor Leonardo Rocha.
If he sticks to his word about there being no plan B, the bilateral ceasefire will be lifted and the war will resume, our correspondent says.
Opposition to the peace accord was led by influential former President Alvaro Uribe. He argued that the government was treating Farc too leniently.
He said that if the 'no' vote prevailed, the government should go back to the negotiating table.