The electricity labour union demonstrated on Wednesday against Energy Minister Siri Jirapongphan's decision to revoke the environmental impact assessment studies for two contentious new coal-fired power plants.
The minister overstepped his authority in making the decision himself, hoping to end the conflict with villagers in Krabi and Songkhla, according to the Electricity Generating Authority of Thailand labour union.
Such a decision should have been made only by the prime minister or the cabinet. They set policies on electricity management in the two southern provinces, union chairman Sirichai Maingam said.
Egat staff wore black clothes at their one-day protest outside the ministry in Bangkok on Wednesday, when the union petitioned Mr Siri to consider whether he had exceeded his authority.
Last week, according to the Egat union, Mr Siri signed an agreement with the villagers, promising his ministry will withdraw the projects' Environmental Health Impact Assessment, to end their rally against the new power plants' construction.
The plants' opponents, who have no confidence in proposed measures to curb the adverse effects, demonstrated near Government House in Bangkok last month before intensifying their call for the cancellation of the power plants with a hunger strike. The protesters camped in front of the UN branch office on Ratchadamnoen Avenue.
They finally ended their protest after the Energy Ministry agreed to make a new "strategic" study to determine whether the power plants should be built.
Mr Siri later said that large new power plants are not urgently needed in the southern region over the next five years.
He said the government also wants to revise its power development plan for 2015-2036, the PDP 2015, to find the real needs of each region, especially in view of the decision to build a modern industrial hub in the East, known as the Eastern Economic Corridor.
PDP 2015 supports the use of coal-fired power plants to ensure adequate electricity supplies across the country.
During the union protest on Wednesday, Mr Sirichai also petitioned the ministry to carefully consider the increasing future demand for electricity, and at the same time the differing opinions of local people.
Any decisions should be appropriate and in line with regulations, he said.
"The Egat union doesn't want to quarrel with NGOs opposing the projects. We respect their views," Mr Sirichai said.
"But we are suspicious of the energy minister's use of his authority."