Nepal's former Maoist rebels have abandoned efforts to form the country's next government, party officials say.
Their decision follows their failure on Monday to get the candidate they were supporting elected as first president of the new republic.
The Maoists were expected to lead the new government as they won most seats in April's elections to a new constituent assembly.
But they say they are being blocked by an alliance of rival parties.
Differing ideologies
"Now we'll not go (in)to the government," the Maoists' 53-year-old leader, Prachanda, was quoted as saying after meeting party leaders.
Correspondents say that the decision by the Maoists, who won one third of the assembly seats, to go into opposition will plunge Nepal into yet more political instability.
The BBC's Sushil Sharma in Kathmandu says that there is now concern that the country may be governed by a shaky alliance of parties with differing ideologies while the Maoists - with their capacity to bring supporters onto the streets - will be watching form the outside.
On Monday, the Maoists' presidential candidate, Ramraja Prasad Singh, lost a run-off in the constituent assembly to Nepali Congress party candidate Ram Baran Yadav by 282 votes to 308.
The vote to decide the presidency was the first major decision by the assembly since lawmakers decided to abolish the 239-year-old monarchy and declare a republic, part of a peace process that ended a decade-long civil war with Maoist insurgents.
The increasingly unpopular monarchy was abolished in May, after a vote in the Maoist-led assembly.
Before then King Gyanendra had appointed a series of prime ministers before sacking the government and assuming complete control in February 2005.
Weeks of demonstrations by hundreds of thousands of people secured the end of direct palace rule in April 2006.
In the same year the Maoists, who had been fighting for a communist republic, declared an end to their insurgency.
Thousands of people from the government and rebel sides died during the decade-long conflict.
By December 2006, seven parties, including the former Maoist rebels and the government party, agreed to abolish the monarchy.
PHOTO CAPTION:
Nepal map
BBC