TDRI president Somkiat Tangkitvanich said the deals would benefit the company but not the country, which already lacks competition in telecoms and other sectors.
If the two deals go through, Thai consumers and small business owners would have even fewer options and be more vulnerable to exploitation by big corporations, he added.
Somkiat expressed his opposition on Facebook, saying he was concerned that the telecoms market will follow monopolistic patterns of other Thai markets such as movies, retail and hospitals.
Both poor and middle-class consumers would suffer, he added.
"Thailand's economy now resembles that of the United States circa 120-150 years ago, when oil companies, railroad companies, and large corporations merged, resulting in widespread exploitation of the people. People couldn't take it any longer, leading to the ‘Progressive Era’ a widespread anti-monopoly movement," said Somkiat.
He urged the National Broadcasting and Telecommunications Commission (NBTC), Trade Competition Commission (TCC), the Cabinet and the judiciary to raise the criteria for mergers and acquisitions, warning that failure to do so would exacerbate economic and social inequality and jeopardise Thailand's sustainability.