Ivan Marquez, a senate candidate and senior member of the political party formed by the former Revolutionary Armed Forces of Colombia rebels, told reporters that party members decided not to field a candidate after Londono underwent open heart surgery on Wednesday.

Since the peace deal struck with the government of outgoing President Juan Manuel Santos in late 2016, the FARC gave up its half-century armed struggle and became a political party keeping the same acronym.

Colombia's presidential election is scheduled for May 27, with a possible runoff vote set for mid-June.

Surveys showed that Londono -- candidate for the Common Alternative Revolutionary Force -- had just one percent voter support.

Marquez however said his party is not dropping out of the legislative elections set for Sunday.

Under the peace accord the former rebels are guaranteed at least 10 of the 268 congressional seats up for grabs in the March 11 election. They can gain more, but they must campaign for them.

- Conservative ahead in polls -

Londono suffered a heart attack last week at the end of his daily exercise routine. He has had serious health scares before: in 2015 he had a heart attack in Cuba while negotiating the peace deal, and in July 2017 he suffered a minor stroke.

Doctors this week also found that Londono suffers from "a chronic pulmonary disease," as well as arterial brain blockage.

"His recovery will take several weeks," FARC Senate candidate Carlos Antonio Lozada said.

The ex-rebel leader however is undergoing a "satisfactory" recovery from heart surgery, Marquez said.

Marquez did not say who -- if anyone -- the FARC could support in the presidential election.

An endorsement could be a kiss of death, given the FARC's abysmal approval rating due to crimes committed during the long internal conflict.

Marquez reiterated his party's call for a "government of transition" that would guarantee the implementation of the peace agreement.

"Not participating directly in the presidential race does not mean that we will refrain from voicing our opinion of the other candidates," Marquez said.

Londono's vice-presidential candidate, Imelda Daza, said that since the peace agreement was signed more than 50 ex-rebels or ex-rebel relatives have been murdered.

Under the peace accord, the FARC -- once the most powerful rebel army in Latin America -- disarmed its 7,000 fighters in order to join the legal political process, agreed to confess to wartime crimes and pay reparations to victims.

This infuriates many Colombians, in particular the right wing, which is vowing to win the presidential election and amend the peace deal.

Current polls show conservative Ivan Duque of the Centro Democratico party leading the presidential race. His party is led by former president Alvaro Uribe, a fierce opponent of the FARC peace deal.

Duque is followed by leftist Gustavo Petro, and former Medellin mayor Sergio Fajardo.