It may mean that it has died. Dig it out with as much root as possible to prevent the fungus infecting any other woody plants.

It is a bigish tree, and looks healthy enough, perhaps I should just wait and see? Thanks for you quick reply.

You may need to get a tree surgeon to look at it as it could become dangerous. They could probably also confirm if it's a killer fungus or one that feasts on dead trees.

What colour are the mushrooms? Or is it bracket fungus? If you're not sure, a photograph would be helpful.

I have to get batteries for my camera, and then I'm away for a few days. Its such a relief to get quick replies for this problem, I'll try to take the picture tomorrow. Many thanks.