Who killed morgause?

In traditional Medieval romance versions, Gaheris kills his mother when he finds her in bed with Lamorak, her much younger lover. Gaheris is incensed that his mother loves the son of Pellinore--who had killed King Lot many years before. Lot being Morgause's husband and the father of 4 of her sons, including Gaheris. (Morte d'Arthur and Vulgate cycle)

I believe TH White is the first author of modern Arthurian fiction to change this and has Agravaine guilty of matricide by beheading her.