GlaxoSmithKline has confirmed 330 jobs will be cut at its Cumbria factory.
Shares in GlaxoSmithKline recently fell by about 8%
The drugs firm, which employs 537 staff at its Ulverston site, made the announcement on Thursday.
GSK said the job cuts would be made over the next two years. It blamed increased competition and the loss of patents on some of its products.
Union bosses said it was a "body blow" to the local economy. Local MP John Hutton is due to have talks with management on Friday.
TGWU spokesman Steve Mashiter said: "What with rising prices of things like petrol, gas and electricity this has come as a body blow to a town where there is no other large manufacturing companies.
"Many of these people have specific skills dealing with the manufacture of drugs such as antibiotics and there are no other jobs like that about."
Shares in GlaxoSmithKline recently fell by about 8% after the pharmaceuticals giant said 2008 earnings would be hit by declining sales of a key drug.
In a statement GSK said: "The current position of rising site costs and decreasing competitiveness is no longer sustainable.
"GSK believes that the proposed scale of restructuring, whilst deeply regrettable, is essential if we are to create a manufacturing business at Ulverston with the potential to compete in the future."