Trade union Unite represents some of the 673 employees at the pharmaceutical giant's Southdown View Way site.
Staff were called into a meeting at 1pm this afternoon to be told some of the work carried out at the Worthing branch would be transferred out of the town after a review of 'global antibiotics manufacturing operations'.
A GSK spokesman confirmed it would result in redundancies and employees have said they expect the losses to top 250.
Unite's regional officer, Mick Pollek, said: “News of potential jobs losses is deeply distressing for members and comes totally out of the blue. These are good quality jobs and Unite will be scrutinising the business case for any potential redundancies while fighting to safeguard jobs.”
A GSK spokesman would not confirm the specific number of redundancies planned, but said exact figures would be decided following a consultation process.