Up to 800 jobs are under threat in Devon and Cornwall as part of a reorganisation by a china clay company.
The company said it was consolidating its operations
Imerys is blaming high energy prices, a weak dollar and strong competition from overseas causing significant losses on sales in Britain.
Imerys said the changes were necessary to protect the future of the business.
Up to 85 jobs will be lost in Devon, and the rest in Cornwall. The company said that a consultation process would be started with unions on Friday.
The company is to close its mining, refining and drying operations in Devon, and consolidate its operations for ceramics and performance minerals in Cornwall.
It operates seven china clay pits in a 25-square mile area around St Austell; and one pit north of Plymouth in Devon, at Lee Moor, which is earmarked to close.
It will stop manufacturing glossy paper coating material in Cornwall and instead focus on the production of paper filler clay. Its production capacity of coating material in Brazil will be expanded accordingly.
The 800 redundancies amount to almost half of the French company's 2,000-strong workforce in the region. But most of the changes will not take place before the fourth quarter of 2007.
An Imerys spokesman in St Austell, Cornwall, said that the company was "hugely saddened" by the "devastating news".
The firm contributes tens of millions of pounds to the local economy and the news would be a "big blow to the local economy", the spokesman said.
But the company added that it was to invest £25m in Cornwall over the next two years to secure its operations there.
Truro and St Austell MP Matthew Taylor said: "I am doing all I can to change this."
He said: "I'm shocked and devastated, as I think every employee of the company will be, and as the whole of the china clay community will be.
"I've already told the company that I want to meet them this week, and I know the unions will be doing what they can."
South West Devon MP Gary Streeter said he was also "deeply disappointed" at the news.
Imerys said it would support any staff made redundant and provide financial advice and counselling for all employees
The company is the world's largest producer of china clay, operating in 250 locations in 43 countries.
The china clay industry in Cornwall was started 260 years ago this year by English China Clays, which was taken over by Imerys in 1999 for £756m.