The road measures between 12 and 20 feet in width, the Rainforest Protectors Organization said in a letter to the President.
It pointed out that the area is facing a massive threat of a landslide due to this construction.
The road will run through Pitadeniya in Deniyaya to Lankagama, area residents and officers involved in the project said.
The relevant authorities have not been informed about the construction of a road through the Sinharaja forest, according to the organization.
Soil that is removed during construction is likely to be deposited in the Gin river during the construction that is likely to block waterways, the letter added.
(Except for the headline, this story, originally published by newsfirst.lk has not been edited by SLM staff)