The project in Sampur in the eastern port district of Trincomalee is NTPC's first overseas coal-fired power project.
The unionists, who have banded together as The Alliance Against Corruption (AAC), say the 500 MW power plant would cause a loss of over rupees 760 million a year and therefore would become a public burden.
Saman Ratnapriya, a leading AAC figure, said that Sri Lanka must annul the agreement entered with the NTPC of India.
"This deal signed by corrupt officials lacked transparency. Even the then minister of power and energy was not aware of its nature," Joseph Stalin, a joint AAC convenor, said.
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The joint venture between National Thermal Power Corporation and the Ceylon Electricity Board is to commence in July, Sri Lankan minister of power and energy Pavithra Wanniarachchi has announced.
The agreement for the USD 500 million project was signed in 2011.
The AAC claims Sri Lanka wants to go ahead with the disadvantageous project as means to please New Delhi.