BOGOTA (Reuters) - Colombia's principal opposition party, led by former President Alvaro Uribe, announced on Wednesday it would back a 'no' vote in a plebiscite on a peace deal with Marxist FARC rebels that would put an end to five decades of war.

The right-wing Centro Democratico party has sharply criticized the ongoing peace talks with the Revolutionary Armed Forces of Colombia (FARC) launched by the government in late 2012. They argue that too many concessions have been granted to the rebels and demand they go to jail for crimes committed during the conflict.

"We can only say yes to peace by voting no to the plebiscite", Uribe, president from 2002 to 2010, said at an event to announce the decision.

"Approving an illegitimate plebiscite is equivalent to accepting total impunity, which instead of dissuading crime consecrates it as champion and makes an example for more, newer violence."

The constitutional court ruled this month that a plebiscite could be held to legitimize any deal and that at least 13 percent of the electorate would need to agree for it to be approved.

The Centro Democratico had also considered encouraging Colombians to abstain from the vote in the hope it would not reach the quorum set by the court.

Santos' decision to ask Colombians to respond to one question with a yes or no answer drew criticism from Uribe, now a senator, who argued that it does not allow voters to reject specific aspects of the peace agreement.

The government and FARC have been hammering out accords in Havana on agricultural reform, FARC participation in politics and dismantling the illegal drugs trade, as well as on other accords that Santos hopes will allow the rebels to hand in their weapons.

(Reporting by Julia Symmes Cobb and Luis Jaime Acosta; Writing by Helen Murphy; Editing by Bill Rigby)