The pharmaceutical company GlaxoSmithKline (GSK) is to close its factory in Kent in 2013 with the loss of hundreds of jobs.
About 620 people are expected to be affected when the Dartford site closes.
The firm, which employed 3,000 people at the site 10 years ago, blamed the closure on the termination of patents.
Its patent for the production of an epilepsy drug has just expired and another for a treatment for herpes is due to expire next year.
Dartford MP Dr Howard Stoate said: "They've employed a huge number of people, they've been a major force in Dartford for good and I think this is going to be a big blow to the area.
"They've been major training employers, they really have been a good company and I think this is going to be a really sad day for the region."
The loss of patents mean that cheaper generic versions of their products can be manufactured elsewhere.
GSK said the two drugs accounted for 60% of its manufacturing at the Dartford site and it was now consulting with unions over the job losses.
Site manager Joe Foley said: "We're not configured correctly for the new generation of products that are coming along."
Asked if the 120-year-old factory could be adapted, he said: "It would be very, very expensive. It would be long and drawn out and we have alternative facilities around the globe to do those particular presentations - vaccines and bio-farms."
The firm, which is the world's second largest drug-maker, announced in July that its Sussex site would close in 2011 with the loss of 493 jobs.
The factory's closure has been blamed on the loss of two exclusive patents
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