Arjun Narsingh KC, a senior leader of the Nepali Congress party, the country's biggest party, said: "The interim constitution will be declared by the present house of representatives on January 15 when the Maoists will be included in a new legislature."
A 330-member interim parliament would include 209 members of the existing parliament, 73 named by the Maoists and 48 members of the public selected by the two sides through consensus.
Krishna Bahadur Mahara, the chief rebel negotiator and Maoist spokesman, said: "We will set up an interim legislature which will endorse the constitution on the same day."
In November, the government and the Maoists signed a landmark peace deal that formally put an end to a decade-old insurgency in which more than 13,000 people have died.
"The Maoists will be included in the government after the UN completely takes up the arms monitoring"
Arjun Narsingh KC, Nepali Congress party
Under the deal, the Maoists will confine their fighters to camps and lock weapons in UN-monitored storage facilities before elections planned in June for a special assembly.
After the special assembly election, a permanent constitution will be created and the fate of the monarchy, which the Maoists want abolished, will be decided.
The peace deal also envisages the rebels joining an interim government in the run-up to the vote.
"The Maoists will be included in the government after the UN completely takes up the arms monitoring," the Nepali Congress party leader said.
UN observers will be ready to register the arms held by the Maoists on January 15, but it has not set any timeframe for the full monitoring mission to begin work.