An announcement is expected later on Wednesday, after nearly four years of negotiations in Cuba.
The two sides singed a bilateral ceasefire in June, paving the way for a final agreement.
“Today I hope to give historic, very important news to the country,” said Mr Santos.
Farc’s leader Timoleon Jimenez, known as Timochenko, has tweeted that “at 18:00 in Colombia (24:00 BST) we will announce the end of the talks and and the conclusion of the [peace] accord”.
“From Havana, I want to share with the Colombian people my satisfaction of having reached this point,” he said on a video posted on Twitter.
The left-wing rebels have been fighting Colombian government since 1964. It is the longest-running conflict in Latin America
Over the past four years, peace negotiators have managed to reach agreement on six areas: land reform, the Farc’s involvement in drug trafficking, justice for the victims, disarmament, future political participation and implementation of the accord.
Once an agreement is announced, Mr Santos will need Congressional approval to hold a popular vote to endorse the terms of the deal.
A vote is expected to take place in October.
Former President Alvaro Uribe is leading the campaign to get the agreement rejected.
The peace talks are aimed at getting the Farc to renounce to more than five decades of armed struggle and join the legal political process.