Stella McCartney was "devastated" to watch her wool suppliers mistreating sheep.
The fashion designer has immediately ceased buying material from Patagonian farms in the southern regions of Argentina and Chile after she was presented with video footage from PETA, which showed scenes of cruelty toward the animals, and insisted she will look to source vegan wool.
She said: "As a designer who built a brand on not using leather, fur or animal skins in its designs, I can't tolerate it! I am devastated by the news but more determined than ever to fight for animal rights in fashion together and monitor even more closely all suppliers involved in this industry to end all innocent lives.
"We are also looking into vegan 'wool' as well, in the same manner we were able to develop and incorporate high-end alternatives to leather and fur over the years."
The 43-year-old design icon began working with the farmers as part of a project to help maintain the area of land where the sheep graze.
But Stella says the farm where the sheep were allegedly mistreated is "one too many".
She wrote on Instagram: "It was born as an amazing initiative to help protect a million acres of endangered grasslands in Patagonia whilst looking after the welfare of animals. Unfortunately, after conducting our own investigation in Argentina, following a very distressful viewing of footage provided by the great guys at officialpeta, we found out that one of the 26 ranches we used source sustainable wool there, mistreated its sheep. It is one too many."
Stella's father Sir Paul McCartney is a committed PETA supporter and vegetarian as was her late mother Linda McCartney, who famously released her own range of meat-free foods.