A Farc negotiator told Colombian broadcaster Noticias Uno "there won't be a signing of the final agreement on 23 March".
He blamed the government for the delay.
The two sides have been holding peace talks for three years to end more than 50 years of armed conflict.
Senior Farc negotiator Jesus Santrich told Noticias Uno that the delay had been caused by government negotiators "changing the rules of the game".
The deadline of 23 March 2016 for the signing of a final agreement had been set by President Juan Manuel Santos on 23 September.
That day, the president travelled to Cuba to announce that the two sides had reached agreement on the issue of transitional justice, one of the thorniest on the agenda.
He and Farc leader Timochenko shook hands at the conference centre in Havana, where the talks have been taking place since November 2012.
A day later, however, another Farc negotiator, Ivan Marquez, was already casting doubt on the viability of the six-month deadline to sign a final agreement.
On Sunday, Jesus Santrich re-iterated those doubts.
He blamed government negotiator for the delay saying they had gone back on points which had already been agreed.
"That was a closed deal," Jesus Santrich said of the agreement on transitional justice.
"And then we had to discuss it twice more, and they [the government] came up with this made-up theory that this was just a draft," he said.
The government maintains the deal on transitional justice reached on 23 September only covered some points and that a comprehensive deal was only reached on 15 December.
The next item on the agenda will be the disarmament of the rebels.
An estimated 220,000 people have been killed as a result of the 51-year-long conflict and more than six million have been internally displaced.
Colombia's largest rebel group, the Farc, says it will not sign a final peace agreement with the government by 23 March 2016 as both sides had previously announced.