The CPN-UML of former prime minister KP Sharma Oli, viewed by many as pro-China, raised objections over the letter allegedly carried by Deputy Prime Minister Bimalendra Nidhi, who was on a visit to India as Prachanda's special envoy from August 17 to 22.
"There are reports about sending a letter by PM Prachanda to Prime Minister Modi through the Deputy Prime Minister that discusses matters of our internal issues including Constitution amendment," the Communist Party of Nepal (Unified Marxist-Leninist) alleged in a statement after the conclusion of its 43rd Central Committee meeting here.
The main opposition party demanded the government to "make public the reported 'letter'".
It warned the government "not to do anything that would undermine the practice of shaping the future of Nepalese people by themselves, and the dignity of Nepalese people."
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Some reports have suggested that Nidhi, during his meeting with Modi, discussed issues relating to the agitation by Madhesis - who share strong cultural and family bonds with Indians - and amendment to the Constitution to address their demands, including more political representation.
The Constitution was promulgated in September last year.
"Constitution is a dynamic document, which is subject to amendments from time to time. However, the way in which the matter was discussed at some specific place for amendment to the Constitution is objectionable and absurd," the party said.
"The fundamental law of the country cannot be amended on the basis of specific concern shown by any other country. This will only complicate the situation and only creates new problems," it said.
Prachanda, who is also the Maoist chief, assumed office earlier this month for a second term as Prime Minister.