Ibobi was a conspicuous invitee to Wednesday's swearing-in ceremony of the Bharatiya Janata Party-led coalition ministry in Imphal.
Ibobi said that since his party was not given a chance to form the ministry, it will remain a responsible watchdog of the coalition government.
Manipur has had no vibrant opposition for several years.
Governor Najma Heptullah, who invited the BJP leader N. Biren Singh to form the ministry, said, "The BJP has the requisite numbers. It was in the interest of Manipur to have invited the BJP. Political stability and development are the issues."
Ibobi, who had claimed majority support, could not physically produce the supporters of the Congress before the Governor.
In the March elections, the ruling Congress bagged 28 seats in the 60-member House. The BJP, which had opened accounts in 2015 after a gap of 15 years by winning two seats in the by-elections, got 21 seats.
The Naga People's Front and the National People's Party got four seats each. All India Trinamool Congress, the Lok Janashakti Party and Independents got one seat each.
All the non-Congress parties and the lone Independent are supporting the BJP now, and the coalition has the total strength of 32 as against 28 of the Congress.
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