The Bord Gáis Group of Unions is criticising the Government’s decision to sell the company to a consortium of private firms.
SIPTU says its members should not have found out about the sale through the media.
Spokesperson Oliver McDonagh says workers felt misinformed.
"Obviously we're very disappointed that the sale is going to go ahead - the unions have been opposed to the sale from the outset," he said.
"We had accepted before that it was going to go ahead, but then it was off again - and we just get informed through the media."
SIPTU is also seeking assurances on job security for staff at Bord Gáis.