The campaign team of leftist Gustavo Petro, leading in opinion polls in Colombia's presidential race, on Monday demanded an "immediate" audit of vote-counting software less than two weeks before the first election round.
Electoral judges have already ordered a software review after discrepancies arose in the counting of votes cast for a new parliament in March, but none has taken place.
The final tally of the March ballot for the Senate and House of Representatives showed 400,000 votes for Petro's leftist coalition that had not shown up in provisional results published on election day.
Ex-guerrilla Petro's "Historical Pact" alliance obtained extra seats in parliament as a result, ending with 45 in the two houses combined.
This made it the biggest grouping along with the Liberal Party, though numerous legal challenges have held up the final, official allocation of seats.
Colombia's right-wing coalition and the incumbent Democratic Centre Party lost seats between the two counts, and challenged the final result.
Observers have expressed concern about possible unrest should the outcome of Colombia's deeply-polarised presidential election also be contested. The first round will take place on May 29, with a likely run-off on June 19.
After the March debacle, the country's National Electoral Council instructed the body in charge of the vote count — the National Civil Registry — to arrange an "international audit" of vote counting software.
But in a letter published Monday, Petro's coalition said it was "concerned" that the registry had not yet appointed an auditor.
It urged registry boss Alexander Vega to act "immediately" in order to "give peace of mind to political forces and citizens."
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Voting is done on paper in Colombia, but the results are processed and transmitted via two digital programs designed by private entrepreneurs.
Civil society organisations have questioned the transparency of these programs and requested a review of their source code.
Petro's campaign also complained after the mayor of Medellín, Colombia's second-largest city, was suspended after implicitly supporting the leftist candidate's run.
Public officials in Colombia, even elected ones, are not allowed to publicly take political sides.
Mayor Daniel Quintero was suspended last week pending a disciplinary probe into claims of "repeated intervention in political activities."
He was replaced by an interim mayor appointed by the outgoing right-wing President Ivan Duque.
Petro, 62, has denounced a "coup d'etat in Medellín" and urged the Inter-American Commission of Human Rights to intervene.