Tunisia's interim parliament was set to pass a much-delayed new constitution Sunday, after the failure to form a caretaker cabinet showed deep political discord.
The Constituent Assembly's lawmakers have already vetted the new document line-by-line after painstaking negotiations on issues such as women's rights and the role of Islam.
The resulting fundamental law, which has been in the works for two years, is a compromise between the ruling Islamist Ennahda party and the liberal opposition.
Lawmaker Karima Souid said the parliamentary session during which the new constitution is expected to be approved was postponed from the morning to the afternoon.
The vote was now expected to start at 1500 GMT.
The charter needs the support of two-thirds of the 217 assembly members to pass.
But in a sign that Tunisia's political class remains deeply divided, the technocrat prime minister chosen last month to steer the country to fresh elections announced after missing Saturday's deadline that he had failed to muster sufficient consensus for a new caretaker cabinet.
Mehdi Jomaa had been expected to submit his line-up to President Moncef Marzouki Saturday afternoon but gave a press conference shortly after midnight to say he would not.
"I chose not to do it in order for a consensus to be reached," he said. "I may be asked again (to form a new cabinet), or it could be somebody else."
Jomaa, the industry minister in the outgoing government, was tasked with ending the crisis which has gripped Tunisia since the assassination last year of two opposition MPs by suspected jihadists.
Several Tunisian media outlets said the main sticking point in the negotiations for a new government was the identity of the interior minister.
Some opposition groups want the current minister, Lotfi Ben Jeddou, removed because he served in the government led by Ennahda, which has the largest bloc in parliament.
But others, not only Ennahda supporters, argue that a volatile security situation across the North African country means that continuity is needed at the interior ministry.
"We aren't far from a solution, talks will continue," Jomaa said.
Ennahda's veteran leader Rachid Ghannouchi hailed the draft charter due to be passed Sunday as a "historic achievement" which he said would enable the establishment of the first democracy in the Arab world.
Under the new constitution, executive power is divided between the prime minister, who will have the dominant role, and the president, who retains important prerogatives notably in defence and foreign affairs.
Islam is not mentioned as a source of legislation, although it is recognised as the nation's religion and the state is committed to "prohibiting any attacks on the sacred," while freedom of conscience is guaranteed.
Approval of the constitution is seen as a key step in Tunisia's political transition, more than three years after long-ruling dictator Zine El Abidine Ben Ali was ousted by the first popular uprising of the Arab Spring.
The vote, initially announced for Saturday, was pushed back until Sunday to allow lawmakers to reform the rules of the confidence vote.