The two leaders met Chief Minister and BJD president Naveen Patnaik at his residence here before joining BJD.
The move would go a long way in ensuring speedy development of poverty-stricken KBK region in Odisha, they said.
Patnaik told reporters that the Congress leaders had formally joined BJD. "I am delighted ... They will be a tremendous asset to the party. We will work jointly for development."
Singh said he had resigned from the primary membership of Congress and the letter has been sent.
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While Singh represents Narla constituency in Kalahandi district in the outgoing assembly, Sai was MLA from Brajrajnagar segment in Jharsuguda district.
Sai said they decided to quit Congress as they felt that several leaders, including them were being ignored by the party which was functioning in an arbitrary manner.
Their resignations from Congress came after days of speculations in political circles about Singh quitting the party and joining BJD.