This is a lovely example of Dryad's saddle, one of the many types of bracket fungi to be found.
The bracket shaped fruiting structures appear throughout the spring, summer and autumn. Some of the bracket fungi favour particular hosts. Dryad's saddle is partial to deciduous trees, in particular elm, beech and sycamore. The brackets can grow up to 60cm in diameter and grow on the trunk or large branches of trees.
Unfortunately, like all parasites, they live at the detriment of their host. Bracket fungi cause white or brown rot in the heartwood, which considerably weakens the tree. Trees can become hollow and weak and eventually fall.