Ex-army officer Ollanta Humala will face former President Alan Garcia in Peru's presidential run-off on 4 June, election authorities have confirmed.
Ollanta Humala, left, and Alan Garcia seem likely to go to a run-off
Mr Garcia had been in a close run for second place in the first round with conservative candidate Lourdes Flores.
After weeks of counting, the board said his lead over her was now irreversible.
With all but a handful of votes counted, Mr Humala leads with 30.7%, followed by Mr Garcia on 24.3% and Ms Flores on 23.8%.
The election campaign was dominated by the rise of Mr Humala, a nationalist, but polls do not suggest he would be certain to win a run-off.
The narrow gap between the three main contenders has delayed final results from the 9 April election from being released.
With no candidate passing 50% support, the two leading candidates will proceed to a second round.
The remaining two candidates both lean to the left, confirming a recent left-wing trend in elections in Latin America.
Mr Garcia presided over Peru from 1985 to 1990, during a period of hyperinflation and a bloody insurgency by Shining Path guerrillas.
But he has promised to create jobs and to divert more of the country's mineral wealth to the poor by taxing mining firms' profits.
Mr Humala first came to public attention when he led a military rebellion against the government of Alberto Fujimori in 2000.
An ally of Venezuela's Hugo Chavez, he has also promised to redistribute the country's mineral wealth, and opposes a free-trade agreement with the US.
Despite Mr Humala's first-round lead, analysts predict most of Ms Flores' supporters would be likely to turn to Mr Garcia in a run-off.
A Datum poll published on Tuesday suggested Mr Garcia would win a run-off against Mr Humala, with 54% against 46%.