A diplomatic source and an aide to the one-time ally of Jean-Bertrand Aristide, the ousted president, said 85,000 "blank votes", in which no choice was made among the 33 candidates competing in the poll on 7 February, had become the focus of efforts to resolve the disputed result.
The source said Brazil, which heads the UN peacekeeping force in Haiti, and Chile were leading efforts to resolve the impasse.
The blanks, amounting to 4.7% of the total, were included in accordance with the law and reduced the final percentage allocated to each candidate, helping to keep Preval below the simple majority he needed for a first-round win.
Preval's share of the vote so far stood at 48.7%, triggering protests by his supporters and a claim by Preval of "massive fraud". Preval, opposed by the same wealthy elite that helped to drive Aristide into exile two years ago, would have 51% of the vote if the blanks were discarded.
Jacques Edouard Alexis, who served as prime minister for a time when Preval was president between 1996 and 2000, said: "We are asking the authorities not to consider the blank votes because they are evidence of fraud."
"We are asking the authorities not to consider the blank votes because they are evidence of fraud"
Jacques Edouard Alexis, a former prime minister of Haiti
Blank votes are a common way to register a protest vote in established democracies.
However, Haitians doubt that so many of their countrymen walked miles to a polling station and then waited for hours simply to cast an unmarked ballot. The United Nations, which helped oversee the election, has also acknowledged that ballot boxes could easily have been stuffed with blanks.
"The focus now is on the blank votes because nobody believes that these blank votes are real," the diplomatic source said, asking not to be identified.
UN for probe
Brazil wants Preval to be declared the winner.
Thousands protested after burnt
ballots were found dumped
Marco Aurelio Garcia, the chief foreign relations adviser to Luiz Inacio Lula da Silva, the Brazilian president, said: "Considering the existing climate in the country, that would be the best solution."
Haiti's interim authorities have bowed to Preval's demand for a fraud inquiry as thousands protested after charred and smouldering ballots were found on a rubbish dump in Port-au-Prince.
Waving burnt ballot papers and ballot boxes, the protesters chanted, "Look what they did with our votes", as they marched past the US, Canadian and French embassies.
Many came from slums such as Cite Soleil and Belair, where Preval has won the same passionate support among Haiti's poor masses that formed the backbone of Aristide's political power.
Michel Brunache, chief of staff for Boniface Alexandre, the president, said the interim government had asked the provisional electoral council not to publish final election results until an official commission had reviewed Preval's allegations.
"We have people who are angry, who are ready to set the country on fire," Brunache said.
Canada, a major aid donor, said it was troubled but optimistic that the fraud allegations would be dealt with.
The UN Security Council repeated a call for calm and asked for a full investigation.
A distant second in last week's election, with 11.8%, was Leslie Manigat, another former president, whose brief tenure in 1988 was interrupted by a military coup.