Sibanye Stillwater's (SBSW) US platinum group metals operations could ultimately have no viable basis for continued operation if planned productivity and operational changes cannot be implemented, the company said Wednesday.

The company has received a strike notice from the United Steelworkers union covering its Stillwater East mine and Columbus metallurgical facility, with strike action expected to begin Thursday, it said.

Sibanye Stillwater has been negotiating with the union for more than four months on a new agreement that includes proposed changes to work practices and a redesigned performance and incentive plan, it said.

The strike's production impact will depend on its duration, operating arrangements, processing facility availability and the timing of a subsequent restart, Sibanye Stillwater said.

Shares of the company were 1.8% lower in Thursday premarket activity.