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 claims 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.
The BBC reports that more than half of the 19 candidates in the ballot on 28 November want the result to be annulled.
Mr Martelly, known to his supporters as Sweet Micky, was widely expected to go through to the second round.
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 was third with 21%, about 6800 votes short of Mr Celestin.
Trouble in the streets
Trouble erupted almost immediately after the results were made public.
Thousands of supporters of Mr Martelly took to the streets of the capital, Port-au-Prince, setting up burning barricades and clashing with UN peacekeepers.
On Wednesday both the headquarters of the governing party in Port-au-Prince and the international airport were closed because of the unrest.
There was also unrest in several other cities.