Haiti's electoral council says the second round of parliamentary elections will be delayed.
Mr Preval is currently on his first foreign visit as Haiti's leader
Council head Rosemond Pradel said it was impossible to keep to the 19 March date because complaints from the first round were still being dealt with.
This means that in the absence of a parliament, the inauguration of President-elect Rene Preval, set for 29 March, must also be delayed.
Mr Preval won Haiti's long-awaited general elections on 7 February.
He is currently visiting neighbouring Dominican Republic in his first foreign visit as Haiti's leader.
The second round of the legislative polls will pit the two leading candidates from the first round in each of 30 Senate seats and 99 Lower House seats against one another.
But the electoral council has not yet set a date for the polls to take place, blaming the delay partly on demonstrations which took place while first-round vote-counting was underway.
Allegations of fraud led to daily street rallies by Mr Preval's supporters, who accused the electoral council of manipulating the vote.
Mr Pradel, the electoral council secretary-general, said at least a week was needed to process complaints from the first round, which were still coming in.
"I could say that we have just begun to receive claims," she said. "So it is quite impossible to respect the 19 March date."
He said a further 24 days were needed to complete preparations such as producing the ballots, so a date in early April looked possible.