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BOGOTA (Reuters) - Leftist Colombian presidential candidate Gustavo Petro said on Sunday that voting software for next week’s contest has been tampered with, in a bid to aid center-right candidate German Vargas, an allegation the government denied.

Petro, an ex-mayor of Bogota and former M-19 rebel, has long held second place in surveys, behind right-wing candidate Ivan Duque. Vargas is in fourth place.

“The software has algorithm alterations that don’t give a guarantee and could generate a massive fraud,” Petro told journalists, questioning what he said was the absence of a European Union electoral observation mission with the expertise to examine the system.

“We are on the verge of not being able at any time to guarantee electoral transparency,” Petro said, accusing President Juan Manuel Santos and electoral authorities of wanting to aid Vargas, who is an establishment candidate supported by Santos’ party.

Polls are notoriously unreliable in Colombia and many analysts have not ruled out the possibility that Vargas’ strong regional and party support could carry him into a second round.

The national registry, which mounts polling places and releases vote counts, said the accusations were false and added that the European Union electoral mission has been present in Colombia since the beginning of May, participating in technical tests and two voting simulations.

“To say now that the software has anomalies or mistaken algorithms is absolutely false,” registry head Juan Carlos Galindo said, adding representatives from Petro’s party have attended the voting simulations.