"Any sale is going to result in job losses and this is punishment for poor workers," NUM general secretary Frans Baleni told Reuters.
"They are an established platinum operator and any sale will have certain consequences."
Anglo American [JSE:AGL] is set to unveil plans on Monday to dispose of its oldest South African platinum mines.
According to the Sunday Times, this move would reduce the global miner's staff count by a fifth of its total workforce, leaving 20 000 employees jobless.
Anglo, the world's No.5 diversified mining company by market value, signalled in April that it could dispose of at least some of its deep, high-cost platinum mines.
South African gold miner Sibanye Gold [JSE:SGL] has already expressed interest in buying the assets.
However, Amplats chief executive Chris Griffith said on Monday "a number" of suitors had expressed interest in buying them.
He also said on a conference call with journalists that the company would stick to growth and capital spending plans for the South African mines that it is keeping.