"Modi will be the Prime Minister of NDA government under any circumstances," he said in an interview to PTI on board his chartered campaign aircraft.
The BJP leader's categorical assertion came in response to a question about the possibility of NDA falling short of a majority and any new ally insisting on a different leader, other than Modi.
"The country cannot be ruled only by the laws but by a leader who has moral authority. Only a person who has been projected as the PM candidate or CM candidate will have that moral authority," he said.
Singh dismissed as "unnecessary controversy" Modi's refusal to wear a 'skull cap' (worn by Muslims).
"This is an attempt to make an issue of a non-issue in a planned manner by our political opponents. I wear dhoti-kurta, he (Modi) wears kurta-pyjama, you wear shirt-pant. Is this an issue? They (opponents) want to communalise the country," he said.
He endorsed the BJP Prime Ministerial candidate's refusal to appeal to any particular community for votes "There should be no appeal to any caste, creed or religion to vote in a particular manner.
Everyone has the freedom to exercise franchise and it should be free. We issue appeal to all sections of the society without any discrimination," the BJP chief said.
About addressing the concerns of Muslims, Singh said, "We are gradually removing the apprehensions... Their fear will be removed through dialogue."