Profits have been rising at GlaxoSmithKline
The firm is to shut down its base in Speke, Liverpool, in the next two-and-a-half years, transferring production to mainland Europe.
Maria Eagle, local MP for Liverpool Garston, said she was "disgusted" by the move.
But Glaxo senior vice-president for European supply, Dr David Pulman, said the rationale for closing the plant was "convincing".
The pharmaceuticals company makes a type of asthma inhaler at the site.
Unions were first told in June the firm was planning to close the plant following the merger of Glaxo Wellcome and SmithKline Beecham in 2000.
A spokesman for GlaxoSmithKline said the firm was proceeding with the proposals to close the site in 2004.
Staff praised
Dr Pulman said: "Obviously we will do all we can to help Speke staff prepare for the future and we have already pledged £1m for re-training.
"We also recognise how this decision will impact the local community and will be talking with the constituency MP, Maria Eagle.
"We will also talk to the regional agencies about how we can work with them to ease the long-term impacts of this decision."
The company, which made pre-tax profit for the second quarter of £1.7bn, praised the attitude of staff for maintaining production during a period of uncertainty.
'Pretty disgusted'
Ms Eagle said: "I am pretty disgusted with the company. I think the case we have was very, very strong.
"I don't understand why they have done what they have done. I think the workforce has a right to feel let down and I intend to see the workforce when I'm back in Liverpool on Friday.
"There has been a big increase in sales of their products and we think they are going to have to invest somewhere to meet this demand.
There's nowhere cheaper to invest than Speke."
'Cash rich'
Roger Lyons, General Secretary of the Manufacturing Science and Finance union, told BBC News Online: "We are very disappointed that they have chosen to close the site.
"With quality employment at a premium in the area this is a tragedy for Speke.
"The company is cash rich and there will be no problem with them paying decent redundancies, but the region really needed to retain these jobs."
GlaxoSmithKline, the world's second-biggest pharmaceutical firm, reported a 15% rise in profits in July.