The former FARC chief negotiator said the 2016 peace deal is being "disfigured" and that he won't be starting work as a senator on Friday.

In a public letter diffused on Monday, Marquez claimed that "insurmountable circumstances are preventing" him from taking his seat in the upper house of Congress on July 20, as was negotiated in the deal to transform the former left-wing guerrilla movement into a political party.

Marquez gave three reasons for his decision not to take up one of 10 seats reserved for the Common Alternative Revolutionary Force party as stipulated in the deal: the detention and potential extradition of fellow former FARC leader Jesus Santrich, who is wanted by the United States for drug trafficking; the modification of the pact; and the lack of conditions allowing the "transformation of the armed rebellion into legal politics."

"It feels like Colombian peace is caught in the networks of betrayal, and not so much because the pact has not materialized -- which requires time to be put into effect -- but because of the modifications that have been introduced that disfigure the pact," said Marquez in his letter.

He had already announced after Santrich's arrest on April 9 that he was suspending his position as a legislator. According to the peace deal, both he and Santrich were due to take up Senate seats.

President Juan Manuel Santos was awarded the Nobel Peace Prize for his role in agreeing the peace deal signed in December 2016 that ended a half century of war that drew in various left-wing rebels, right-wing paramilitary units, drug cartels and the army.

But he is due to step down on August 7 when the right-wing Ivan Duque will take over.

Upon his election last month, Duque promised to make "corrections" to the peace deal, which he criticized in his campaign for being too lenient towards former rebels.

FARC responded by imploring Duque to show "good sense."

Duque is opposed to former rebel leaders that have been found guilty of crimes against humanity being allowed to serve as congressmen.

The peace deal stipulates that former fighters who confess to their involvement in atrocities can avoid prison.

Some 260,000 people were killed, 60,000 disappeared and 6.9 million displaced during the 53-year conflict.