Election officials in Haiti say they will review the disputed result of last month's presidential election.
There will be an immediate vote recount in the presence of the top three candidates - Mirlande Manigat, Jude Celestin and Michel Martelly - and international observers.
The announcement follows violent demonstrations by supporters of Mr Martelly, the third-placed candidate.
He alleges the count was rigged to deny him a second-round run-off place.
The Provisional Election Council said it had "decided to immediately launch a rapid and exceptional process to verify at the counting centre the tally sheets linked to the 2010 presidential elections".
More than half of the 19 candidates in the ballot on 28 November have been calling for the result to be annulled.
The strongest protests have come from supporters of pop star Mr Martelly, known to his supporters as "Sweet Micky," who was widely expected to go through to the second round.
The results announced on Tuesday night gave 31% to the former first lady, Ms Manigat, with the governing party candidate, Mr Celestin, in second place with 22%.
Mr Martelly came third with 21%, about 6,800 votes short of Mr Celestin.