Tunisia's National Constituent Assembly on Friday said it will next week begin considering the draft constitution, a text developed over two years and delayed by a political crisis.

"From Monday (December 30) deputies will present their proposals (for amendments) on the draft constitution, and on January 3 its examination, article by article, will begin," Mofdi Mssedi, spokesman for the presidency of the assembly, told AFP.

In order to be adopted, the draft must be approved by two thirds of the 217 elected members of the assembly or be submitted to a referendum.

The government headed by the Islamist Ennahda party, its allies and the opposition all want a consensus in the assembly on the charter.

A national dialogue began on Monday, after three postponements, on forming a new government to replace the one headed by Ennahda's Ali Larayedh.

The talks, mediated by the powerful UGTT trade union, focused on when independent Mehdi Jomaa, currently industry minister, will take over as premier.

His nomination to form a government of independents was agreed as a way out of the political deadlock gripping Tunisia since July, when MP Mohamed Brahmi was assassinated by suspected radical Islamists.

According to the powerful UGTT trade union, the main mediator in trying to solve the crisis, the formal resignation of Larayedh's government should come on January 8.

Ennahda, which has been sharply criticised for failing to rein in Tunisia's jihadists since being elected in the wake of dictator Zine El Abidine Ben Ali's 2011 overthrow, agreed in October to step down as part of a political road map brokered by mediators.

Tunisia's politicians have until January 14 to adopt a constitution and form a new government, the mediators said.

January 14 will be the third anniversary of Ben Ali's flight from the country, which set off the Arab Spring.

Last Friday, Jomaa promised to "favour the appropriate conditions for transparent and credible elections, the security of Tunisians and promoting the economy with the aim of emerging from the crisis".