Political parties from southern Nepal withdrew support from the government on Thursday, weakening an already fragile administration and throwing long overdue elections further into doubt.
An alliance of parties representing the country's Madhesi ethnic minority joined the government when it formed last August on the proviso that the constitution be amended to grant them greater political representation.
But the legislation has been deadlocked in parliament for months, angering the parties.
"We had given the government a seven-day ultimatum to address our demands, but they did nothing," Laxman Lal Karna, a leader of the United Democratic Madhesi Front (UDMF), told AFP.
The office of the speaker of parliament confirmed they had received a notice of withdrawal from the Madhesi alliance.
The government had anticipated the rebellion and last week managed to cobble together enough extra support to ensure its brittle coalition retained a majority in parliament.
The Madhesis have vowed to disrupt local elections slated for May -- the first in nearly two decades -- and there have already been clashes between protesters and government forces.
Earlier this month, five minority Nepalis were killed when police opened fire as protesters tried to disrupt a rally by the main opposition party in Saptari, a Madhesi stronghold.
The alliance would "further intensify" its campaign against the long-awaited polls, the UDMF's Karna said.
"The government still has the numbers, but when it was formed it was to amend the constitution with the Madhesis," said Kathmandu-based political analyst Vijay Kant Karna.
"Now a new kind of conflict arises."
The Madhesis, who share close linguistic and cultural ties with neighbouring India, have long complained of being marginalised by the Kathmandu-centric political establishment.
Madhesis staged a months-long border blockade when Nepal's constitution was passed in September 2015, leading to a crippling shortage of essential goods in the landlocked Himalayan nation.
More than 50 people died in clashes before parliament agreed to amend the constitution and redraw federal boundaries, a move that would grant Madhesis greater political clout.
India, which was accused by the previous Nepali administration of fuelling the protests, said it was monitoring the situation.
"It has always been our position... that the political leaders in Nepal could address the constitutional issues through discussion and consultations with participation of every section of the society," said foreign ministry spokesman Gopal Baglay.
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