"We still have a few outstanding issues on agreeing to a common programme. Hence tomorrow's signing of the MoU will not take place," Nirmal Ranjith Devasiri, a convener of the opposition movement said.
Among a series of poll promises for greater democratisation of governance, the Memorandum of Understanding is to contain a provision to abolish the current executive presidential system within six months of the victory of the unity candidate.
Meanwhile, the ruling UPFA coalition poked fun at the opposition parties for their inability to agree to a common candidate despite much media hype and speculation.
"They have been trying over the last 3 years to find a common candidate to challenge the President. They could not do so," Sports Minister Mahindananda Aluthgamage said.
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Main opposition leader Ranil Wickremesinghe, however, said, "We will announce the unity candidate only after the election notice is issued."
Rajapaksa seeks a record third term riding high on his popularity with the majority Sinhala community after successfully ending the country's nearly three-decade-old civil war with the LTTE in 2009.