Mohan Baidhya, leader of the newly created Communist Party of Nepal - Maoist, said Prime Minister Baburam Bhattarai has been dragging his feet over drafting a new constitution.
Baidhya also criticized Bhattarai and UCPN Chairman Pushpa Kamal Dahal -- known as Parchanda -- for not speeding up the integration into the army of thousands of former communist guerrilla fighters since the civil war ended in 2006.
The split throws the immediate future of the Maoist government into doubt ahead of national elections planned for November, according to a report by the BBC.
Baidhya also said his party was against parliamentary politics and that it might not even register with the Election Commission, a report by the Nepalese news Web site Nepalnews.com said.
However, Baidhya held out hope that reconciliation with the main UCPN could be achieved if Bhattarai and Dahal repented their mistakes and came around to the ideological line of the new party, the Nepalnews.com report said.
Baidhya said he wasn't calling for open revolt against the government but didn't rule it out at a later date.
He urged "revolutionary forces" to join the new party which was "a beginning of a new chapter in the country."
Except for naming Baidhya as its chairman and forming a 44-member central committee, the new party hasn't finalized a formal leadership structure, Nepalnews.com said.
The issue of integration of former Maoist rebels into the army has been a longstanding problem for the communist government.
Around 15,000 people were killed and up to 150,000 people displaced during the decade-long civil war in which the Maoist's Communist Party of Nepal wanted to overthrow the monarchy and set up a republic.
But in November 2006 the Maoists joined other political parties in a peace accord, monitored by the United Nations, in an effort to create a more democratic government for the isolated Himalayan Mountain country with India to the south and China to the north.
The accord, signed by Prime Minister Girija Prasad Koirala and Maoist leader Prachanda, allowed the Maoists to take part in government and placed their weapons under U.N. monitoring.
The monarchy was abolished in 2008 but a new constitution and integration of the rebels remain elusive, partly because the UCPN runs a minority government in Parliament. It must get agreement of opposition parties to draw up a new constitution and for the number of rebels allowed into the army.
In December, Bhattarai met opposition leader Sushil Koirala, of the Nepali Congress Party, in an effort to convince him to allow more than 6,500 rebels into the army -- a number agreed the month before, a report by the BBC said at the time.
An estimated 19,000 registered former Maoist fighters are waiting in remote base camps, a report by the on-line newspaper eKantipur.com said.