Rubber chemicals have been made at the site since the 1930s
Management at a chemicals factory have blamed competition from the Far East for the decision to close with the loss of 163 jobs.
The US-owned Flexsys Rubber Chemicals said the closure at Cefn Mawr, Wrexham, would take place over three years and had been "a difficult decision".
The company said Flexsys operations would stop by the end of the year.
The factory has been open since the 1930s and makes specialised chemicals for the rubber industry.
The parent company Solutia Inc has started the 90-day consultation with workers to try to find an alternative.
The company said other "guest" operations on the site were not expected to close until 2010, with the site as a whole shutting in 2011.
Union official Peter Hughes said shocked workers had assumed their jobs were safe for the foreseeable future because the factory had been scaled down several times before.
The factory, known locally as the Ruabon Works, is one of the oldest industrial sites in the Wrexham area.
It has produced chemicals for the rubber industry since the 1930s, and Flexsys Rubber Chemicals Limited took over operations there in 1995.
Manager Simon Howarth said: "Increasing competition from low-cost manufacturers in the Far East has led to this decision to exit those parts of the Flexsys business served by the Ruabon site.
"While our people have worked tirelessly in recent years to ensure a safe and cost-effective operation at Ruabon, Flexsys has been forced to make this difficult decision to secure its wider business.
"In making this announcement, Flexsys regrets the loss of employment and the impact this will have on the lives of its employees at Ruabon and will do all it reasonably can to provide assistance to them.
"The company also recognises that this closure represents the end of an era for a manufacturing site with a long and distinguished history."
In a statement, Wrexham Council's chief executive Isobel Garner and the authority's leader Aled Roberts said: "We are disappointed at this announcement and we understand the impact this will have on employees and the community as a whole.
"We are arranging a meeting with the owners to find out their plans for closure and for the future of the site.
"The rapid response redundancy team will be contacting the employees to offer one to one interviews and offering advice about potential employment and self employment."
Environmental campaigner Janet Williams, who led protests over what she described as hazardous chemicals and processes at the Cefn Mawr works, greeted the announcement with mixed feelings.
"I feel very sorry for those who're being made redundant and the hardships they face, but I hope this will provide an opportunity to develop the area for tourism and leisure, especially with the efforts being made to designate the nearby Pontcysyllte aqueduct as a world heritage site," she said.
"It has never been the perfect location for a chemical works and I hope now that the company will make every effort to clean up the site so that we can move forward. "They have a duty to do everything they can for their workers in helping them to retrain and find new employment.
"They also have a duty to the community as a whole to ensure that the site is left clear of any contamination. This is a golden opportunity for the council and the local community to work with the company to capitalise on this situation."