By Helen Murphy and Luis Jaime Acosta
CARTAGENA, Colombia (Reuters) - A rejection of Colombia's peace accord in a referendum next week would be a "leap into the abyss" as it could intensify the war and cost the country a decade to get Marxist FARC rebels back to the negotiating table, the government's chief peace negotiator told Reuters.
The government and the Revolutionary Armed Forces of Colombia (FARC) will on Monday sign the historic accord to end a war that has dragged on for 52 years and claimed the lives of 220,000 people.
The final obstacle is a single-question vote on Oct. 2 dividing Colombians between those who believe in forgiving the FARC for the national good and those who prefer to see the rebels defeated on the battlefield and jailed.
Most polls see voters approving the deal, which allows FARC leaders to serve alternative punishments - like clearing land mines - and gives some of them congressional seats without election.
"If a 'no' vote wins, it simply means it's all over," Humberto de la Calle told Reuters just hours before President Juan Manuel Santos, 65, and FARC leader Timochenko, nom de guerre for 57-year-old revolutionary Rodrigo Londono, will sign the pact.
"Experience shows that in Colombia when a peace process is interrupted it takes about a decade before it can get back on track."
De la Calle, a 70-year-old lawyer, spent almost four years in Havana negotiating the conflict's end and a return to society for 7,000 rebels. He also participated in failed talks back in 1991.
Santos has said he will take the nation back to war if the accord fails to pass. The FARC gained strength after previous peace processes failed.
"The accord is the best possible, and even with its critics, it creates certainty," said de la Calle, who is rumored as a candidate to replace Santos in 2018. "The leap into the abyss comes from a 'no' so I say vote with conscience."
Also in Cartagena on Monday was influential former President Alvaro Uribe, who along with hundreds of others, raised red, blue and yellow umbrellas and formed a human chain in protest of the accord.
The FARC, which began as a peasant revolt in 1964, will now begin to hand in weapons and transition into a political party. Santos has said he doubts rebels will get much political traction given their strong Marxist rhetoric, but de la Calle urged Colombians to be ready for change.
"Colombians better prepare for a new party with radical positions to enter the field," said de la Calle. "The FARC won't change its ideology."
(Reporting by Helen Murphy; Editing by Andrew Hay)