By Gopal Sharma KATHMANDU, April 16 (Reuters) - Nepal's Maoists on Monday threatened mass protests to press the government to declare the Himalayan nation a republic after the top election body said an election determining the monarchy's future should be delayed. The former rebels, who battled the monarchy in a decade-long insurgency that cost some 13,000 lives, joined an interim government this month under a peace deal envisaging elections on June 20 for a constituent assembly. The assembly will decide the fate of the monarchy. But last week, the country's Election Commission said it was ill-prepared to hold the polls in June and wants more time. "If elections can't be held on time the interim parliament should declare a republic," said Baburam Bhattarai, de facto number two to Maoist chief Prachanda, told Reuters. "And if other political parties agree to this, it will be a new basis for our unity." "If political parties don't agree to our proposal then we will go to the people and organise mass protests," Bhattarai added. He said the June election was the main basis for unity between seven mainstream political parties and the Maoists, who jointly organised street protests last year forcing King Gyanendra to end his absolute rule. "That basis has crumbled now and we should find a new one," Bhattarai said. But the Maoists vowed not to jeopardise the peace deal that ended a decade-old conflict. "We have not taken a decision to leave the government as yet," he added. Prime Minister Girija Prasad Koirala would convene a meeting of the ruling alliance, including the Maoists, to discuss a new date for the election, officials said.