A Flintshire cement works has been ordered by the Environment Agency to stop using one of its kilns following a breach of emission limits for dioxins.
A new £60m kiln is being built at the Padeswood site
The enforcement notice requires Castle Cement's Padeswood works to suspend activities until it can prove it can operate within the limits.
Detailed monitoring has been carried out at the site.
The company said it had voluntarily suspended operations on kiln three until test results were available.
"The Agency takes seriously all breaches of emission limits and we have no hesitation in taking enforcement action against companies to ensure they operate within the conditions of their permit," said Steve Moore from the Environment Agency.
"Our investigations are continuing and we have not ruled out further enforcement action."
Danny Coulston, general manager at the Padeswood plant said the company has voluntarily suspended operations on kiln three.
"The results of some emission data recently reported to the Environment Agency on kiln three are higher than the measurements the agency had taken themselves last year," he said.
"Because of these high results, we have voluntarily suspended operations on kiln three until a re-test is carried out and results are verified. We had always scheduled to close kiln three on 28 March this year," he added.
Last December, Castle Cement was given the go ahead to begin operating a £60m kiln at Padeswood which is expected to come into operation on a trial basis in June.
The kiln is designed to run on Cemfuel, a controversial concoction of waste solvents and chemicals - which protesters fear will pollute the environment.
However, the Environment Agency has issued a permit which demands strict controls on pollution.
But Arnold Woolley, chairman of the campaign group which has been fighting the new kiln development said the decision to suspend the operation of one of the old kilns only reinforced their fears about the plant.
"As yet I am not convinced that their activities will be environmentally safe.
"It's unfortunate that they have not been able to bring the dioxin emission levels below the required limits because it's a scientific fact that there is no known safe level of dioxin intake for the human body," he added.