The deal resolves one of the final points in peace talks between the government and FARC.
Colombia's government and the FARC guerrilla force were to sign a definitive ceasefire Thursday, taking one of the last crucial steps toward ending Latin America's longest civil war.
The breakthrough -- while not yet a final peace accord -- means a permanent end to fighting in a half-century conflict that has killed hundreds of thousands of people in the jungles of the major cocaine-producing country.
Colombia's President Juan Manuel Santos and FARC leader Timoleon Jimenez were scheduled to sign the agreement at 1630 GMT in Havana where peace talks are taking place, negotiators said.
Santos tweeted that he was "on the way to Havana to silence the guns forever."
"We have successfully reached agreement for the bilateral and final ceasefire and the end of hostilities; the laying down of arms; security guarantees and the fight against the criminal organizations responsible for homicides and massacres," the two sides said in a joint statement.
Foreign leaders and officials including UN Secretary General Ban Ki-moon will attend Thursday's ceremony in Havana.
The deal resolves one of the final points in peace talks between the government and the Revolutionary Armed Forces of Colombia (FARC), the country's largest rebel group.
Santos said this week he hopes to seal a full peace deal by July 20.
The Colombian conflict started in the 1960s as a rural uprising for land rights that spawned the communist FARC.
It has drawn in various leftist rebel groups, right-wing paramilitaries and drug gangs over the decades.
The violence has left 260,000 people dead, 45,000 missing and nearly seven million displaced, according to official figures.
Human rights groups say atrocities have been committed on all sides. Many families are still searching for missing loved ones.
The ceasefire "means the end of the longest and most bloody conflict in the western hemisphere and a new opportunity to bet on democracy," said Angelika Rettberg, a conflict resolution specialist at the University of the Andes.
However, the means of implementing a final peace deal remain to be settled.
Santos's government wants a referendum to put a seal of popular approval on the peace after three-and-a-half years of negotiations.
The FARC was reluctant but has recently signaled it may accept such a plebiscite.
The two sides have signed provisional accords on compensating victims and fighting the drug trade that has fueled the conflict.
They are also discussing designating zones where the FARC's estimated 7,000 remaining fighters can gather for a UN-supervised demobilization process.
"The UN is prepared to do whatever it can to strengthen the peace process," its deputy spokesman Farhan Haq said on Wednesday.
"We hope it will lead to a final agreement and the end of this long war."
The United States sent it Special Envoy Bernard Aronson to Havana to represent Washington on Thursday.
US State Department spokesman John Kirby said the United States "hopes the parties will continue to make progress toward a final peace accord."
Although peace with the FARC would virtually end the conflict, other armed groups are still operating in Colombia.
Santos and the country's second-biggest rebel group, the leftist National Liberation Army (ELN), have said they will start peace talks.
However, that initiative has stumbled because of alleged kidnappings by the group.
"The activity of the ELN above all and the criminal gangs means that we cannot yet talk of a complete end to the armed conflict," said Kyle Johnson, Colombia analyst for the International Crisis Group.
"It will be the end of Colombia's biggest armed conflict, but not all of them."