The firm is selling its Zinnat operation to Swiss-owned Sandoz, which will transfer manufacturing to its own plants in Austria.
GSK’s site in Ulverston, Cumbria, and its Zinnat buildings in Barnard Castle, County Durham, are to close by 2025.
The firm said it wanted to “simplify its portfolio” and invest in research and development and new products.
Its other business in Barnard Castle is not affected.
The Ulverston site currently employs 130 staff manufacturing the cephalosporin class of antibiotics under the brand name Zinnat.
There had been hopes a buyer could be found who wanted to maintain manufacturing in the town.
GSK said it had sold its Zinnat arm for £253m ($350m) plus additional payments of up to £108m ($150m) depending on milestones being reached during the transfer, which would take about four years.
“At this time, and in the absence of any other alternative, GSK would propose to close the Ulverston site and to decommission the Barnard Castle Zinnat buildings,” it said.
The deal still has to be given the relevant regulatory approval.
The company said it was “providing support to potentially-affected employees and is committed to supporting the local communities affected by the potential closure of manufacturing facilities”.