Heptulla today invited Singh, who was unanimously elected the leader of the 21-member BJP legislature party yesterday, to form the government.
The invitation to Singh came on a day when four members of the Naga People's Front, an NDA ally, met the Governor and declared their support to BJP for government formation.
Raj Bhavan sources said the swearing-in would take place at one PM tomorrow.
On being asked about how many ministers will take oath along with the Chief Minister, BJP sources here said it was yet to be worked out.
"However, It will be a small ministry and it will have representation from other allies," the sources said.
The BJP has claimed support of 32 MLAs in the 60-member assembly.
Raj Bhavan sources said the Governor wanted to meet the NPF MLAs personally to ascertain whether they backed the BJP in forming the government.
After Goa, Manipur would be the second state among the five where elections were held recently where the BJP would form a government despite not being the single largest party by cobbling together a post-poll alliance.
Congress, which has 28 MLAs, led the tally. Okram Ibobi Singh, its Chief Minister for three consecutive terms, had resigned and staked claim to form the government, but numbers were on BJP's side.
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Heptulla told reporters that her decision to invite the BJP was backed by the fact that it has the requisite numbers.
Asked to comment on the Congress's contention that she should have invited it first because it was the single largest party, she said, "I don't know what allegations they are making. I am a straight-forward person. I go by the book. I know rules and regulations."
She further said, "BJP has the requisite numbers. They are more than 30. It will be helpful for Manipur... I feel it is very necessary that we should have stability in the state.