BOGOTA (Reuters) - Only 40 percent of Colombians will participate in a vote on whether or not to approve a peace deal with the leftist FARC rebels, a Gallup poll showed on Thursday, as negotiations to end more than 50 years of war drew to a close.

The government of President Juan Manuel Santos and the Revolutionary Armed Forces of Colombia (FARC) have been in peace talks for nearly four years, and recently signed a historic ceasefire deal considered the penultimate step to a final accord.

Santos, who has said a final deal may be agreed as early as this month, has promised any accord will go to Colombians for approval in a public vote.

The government, which says 13 percent of the country's 33 million eligible voters would need to participate to make a vote binding, is waiting for the constitutional court to approve the referendum mechanism.

Forty percent of those polled by Gallup said they would definitely participate, while 22 percent said they would definitely abstain.

A further 19 percent were less sure of their participation, but said they would likely vote, while 9 percent said they might not. Eight percent were undecided.

Of the 40 percent who said they would be voting, 70 percent would approve a deal, while 17 percent would not and 13 percent were undecided, the survey showed.

Opponents of the peace process, which include right-wing ex-president Alvaro Uribe, may encourage their supporters to abstain from voting in the hope a referendum would not meet quorum and be declared invalid.

Sixty-nine percent of people in the survey said they backed the peace process, up 17 points from a poll in May. Santos' approval rating was up by 9 points to 30 percent.

The poll, conducted via telephone with 1,200 respondents in major cities including Bogota, Medellin and Cali, had a 3 percent margin of error.

(Reporting by Luis Jaime Acosta,; Writing by Julia Symmes Cobb; Editing by Bernadette Baum)