GlaxoSmithKline (GSK) has confirmed that more than 300 jobs will be lost in Sussex as part of cost-cutting plans.
GlaxoSmithKline has several UK offices
The drugs firm said there would be 180 losses at its Worthing site and 130 at its base in Crawley.
All areas of the sites and all levels of the organisation will be affected, according to GSK.
The company announced on Wednesday that it needed to save £700m annually within three years to help offset falling sales of diabetes treatment Avandia.
GSK employs more than 100,000 people globally with a workforce of 1,140 at its site in Worthing as well as operations in Dartford, Maidenhead and Crawley.
The proposed job cuts will include permanent as well as contract positions and are expected to be completed by the end of 2009.
"We regret the impact of this proposal, but would like to emphasise that today's announcement is based on a review of the forecast volumes and the need to improve the efficiency of our operations," said Ian Holdstock, Crawley site director.
"This is not a reflection of the performance of our employees.
"The site has a good customer service record and has contributed to a number of successful product launches this year.
"This is disappointing news but we are committed to supporting our staff though these difficult times."
GSK also announced that profits in the three months to the end of September had fallen by 4% to £1.88bn, on sales down 3% to £5.48bn.
The firm was hit by competition from generic drugs and falling sales of Avandia, which dropped 38% to £225m.