Tunisia's new constitution will not be adopted until April 2013, six months later than planned, the head of the drafting committee said on Monday, threatening to compound political uncertainties.

"The final draft of the constitution could be put to the vote (in the National Constituent Assembly) at the end of April," said Habib Khedher, a member of the ruling Islamist party Ennahda and MP in the interim parliament.

"I think that is a realistic target," Kheder told AFP.

The government had until now insisted that it would meet the deadline of October 23 for ratifying the new constitution, in order to hold planned general elections in March next year.

The new timetable looks likely to delay those fresh elections and aggravate the political uncertainties already facing Tunisia, which has been rocked by social unrest in recent months.

The government's failure to improve living standards, as well as fears that a rising Islamist tide threatens personal freedoms, have led to strikes, protests and confrontations with the police.

Kheder declined to comment on when he now expected the elections to be held, saying that was "the responsibility of the government."

The constitution's revised timetable will be discussed at a meeting of the NCA on September 3, ahead of the next parliamentary session, he said.

Tunisia's main parties were given one year to draft a new constitution, after elections last October that brought Ennahda to power at the head of a coalition also grouping centre-left parties the Congress for the Republic and Ettakatol.

Opposition MPs were quick to criticise the new hitch to the democratic transition in Tunisia, 17 months after the revolution that overthrew veteran strongman Zine El Abidine Ben Ali and touched off the Arab Spring.

"Can the country handle another such delay? Can Tunisia's fragile economy withstand another delay?" asked Issam Chebbi, MP and spokesman for the Republican Party, when contacted by AFP.

"We will demand a law that fixes a date for the elections," he said, adding that "the longer the transition lasts, the more the chaos will spread."

The new constitution is due to replace the provisional laws that have governed Tunisia since the revolution, and allow fresh elections to take place.

But it has been heavily delayed due to an apparent deadlock over certain key issues, with the Islamists seeking a pure parliamentary system and the other parties wanting important powers to remain in the hands of the president.

The news of further setbacks comes amid heightened criticism of Ennahda by opposition and civil society groups, which accuse the ruling party of increasingly authoritarian and Islamist tendencies.

Another factor behind rising social discontent is the persistence of poor living conditions in certain parts of Tunisia, and high youth unemployment in particular, a key factor behind the revolution.

Explaining the delays to the timetable, Khader said the first version of the constitution should have been presented to parliament at the end of last month, but was never completed, while the final draft should have been ready by October.

The six committees responsible for the six different chapters of the constitution have to submit each article individually for approval by MPs, who can make further amendments.

Only then can the text be adopted by parliament, where it requires a two-thirds majority. Failing this, the draft constitution would be put to a referendum.