"The national government and FARC delegations inform the public that we have successfully reached an agreement for a definitive bilateral ceasefire and end to hostilities," it said.
The announcement heralds an end to a half-century conflict that has killed hundreds of thousands of people in the jungles of the major cocaine-producing country.
The deal would all but end the conflict by resolving one of the final points at peace talks between the government and the Revolutionary Armed Forces of Colombia (FARC), the country's biggest rebel group.
FARC commander Carlos Lozada tweeted: "On Thursday, June 23, we will announce the last day of the war."
Another FARC leader had earlier said a deal was "nearly" agreed with one point still to be settled.
But the later statement confirmed the deal was complete and would be formally announced Thursday at a ceremony with Colombia's President Juan Manuel Santos and FARC commander Timoleon Jimenez.
It said foreign leaders and officials including UN Secretary General Ban Ki-moon would attend the ceremony.
Presidents Raul Castro of Cuba, Nicolas Maduro of Venezuela and Michelle Bachelet of Chile will be among the other leaders present.
Colombian President Juan Manuel Santos said this week he hopes to seal a full peace deal by July 20.
The means of implementation of such a deal remain to be settled.
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Half-century conflict
The Colombian conflict started as a rural uprising in the 1960s.
It has drawn in various leftist rebel groups, right-wing paramilitaries and drug gangs over the decades in this South American state of 49 million people.
It 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 accord covers "the laying-down of arms, security guarantees and the fight against the criminal organizations" accused of fueling the conflict, the statement said.
"This 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.
Demobilization
Santos's government wants a referendum to put the seal of popular approval on its peace effort.
For that it needs the country's constitutional judges to approve a law already passed in Congress.
Peace talks have been underway in Havana since 2012. They got a boost when the FARC declared a unilateral ceasefire a year ago.
The Marxist guerrilla group agreed to remove child soldiers from its ranks as part of the peace deal.
The questions of disarmament and justice for victims make the road to peace and reconciliation a hard one.
The sides are discussing designating zones where the FARC's estimated 7,000 remaining fighters can gather for a UN-supervised demobilization process.
Santos and the country's second-biggest rebel group, the leftist National Liberation Army (ELN), have also said they will start peace talks.
That initiative has stumbled due to alleged kidnappings by the group.
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