This is a fairly large bracket fungi which I have seen singularly on Birch tree trunks or in tiers going quite high up the tree.
Sometimes called the ‘Razorstrop Fungus’ (so named for in the past it has been used as a strap to sharpen razors), it has a smooth leathery upper surface which is pale brown, whilst the rounded margin and underside is white. The underside is soft and spongy, and full of minute spores. Fruit body up to 25cm across.
Seen all year round in birch woods, and is the cause of death of many of these trees. Common and widespread.