Just two weeks from Christmas, around 340 employees have been told they will be directly affected by the phased withdrawal from Aptuit's Edinburgh and Livingston facilities over the next 12 months.
Sources say staff were told of the closure of the 200,000sq ft research and development laboratory in the capital's Riccarton area, where the majority of workers are based, on Thursday.
Aptuit, a drug development corporation, which also has an office in Glasgow and recently established a 20 million facility in Bathgate, said it had been affected by the "worldwide economic downturn". The proposed withdrawal is understood to amount to a halving of the firm's Scottish operation.
One former senior employee at Aptuit's Edinburgh laboratory described the closures as "another body blow to the pharmaceutical industry in Scotland".
A spokeswoman for the US-based firm said that some roles could be transferred to other facilities.
She continued: "Aptuit is proposing a phased withdrawal from our facilities in Livingston and Riccarton, Scotland, over the next 12-month period. Aptuit understands the serious impact that this may have and, if the proposals are implemented, would have on our employees and the community. We will do everything possible to listen to our employees' needs and concerns and we are committed to exploring all available alternatives."
City councillor Ricky Henderson, whose Pentland Hills ward includes Riccarton, said the announcement came at a "dreadful time" for families.
The Labour councillor will table an emergency motion at this week's full council meeting to determine if the local authority can help.