Amanita phalloides (Fr.) Link
An introduced species associated with oak and sweet chestnuts. This species is deadly poisonous.
Cap diam: up to 90 mm. The cap colour can vary from pale yellowish with a green tint to greyish with a brown tint. The stipe has a ring and a swollen base.
It is responsible for most of the mushroom poisonings worldwide. It causes acute liver failure. There is plenty of information on the internet about the symptoms.

