The decision will be taken in a couple of days, the NCP general secretary said.
The National Congress Party (NCP), which won seven seats in the Nagaland assembly elections, will announce its decision to either join the NDPP-BJP alliance or remain in opposition on Monday.
Leaders of the party in Nagaland said the delay in the decision is over the BJP being a partner in the winning alliance. The final call will be taken by party chief Sharad Pawar, they added.
Addressing a press conference, NCP general secretary Narendra Verma said, "This is the result of our hard work in northeast India over the past 10 years. I had travelled across Nagaland for the past four months and especially concentrated on the state's eastern parts. The people of Nagaland have entrusted us with their votes and we will be holding a meeting two days to decide on the leader of the Opposition."
"I have considered the views of the MLAs, and also the opinion of the state party leaders. The final decision on whether to join the government or to remain in the opposition and also the leader of the legislative party will be taken by the high command in Delhi. The decision will be taken in the next couple of days after I apprise national president Sharad Pawar on Sunday," Mr Verma added.
The NCP had fielded candidates in 12 seats of the 60-member assembly for the February 27 elections.
Another Maharashtra-based party, Republican Party of India (A) led by Union minister of state Ramdas Athawale, has won two seats in Nagaland. The RPI's Vinod Nikalje was given the responsibility to handle the party's Northeast activities.