looks a bit like hen of the woods but on a fallen birch trunk. Note it has pores not gills under the cap.
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Some fungi are very poisonous so a mistaken ID could have serious consequences.
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From the top and without seeing the pores it looks like Pleurotus ostreatus or a pale Hohenbuehelia petaloides. However it has pores. What about a young Meripilus giganteus?
I was wondering more about Grifola frondosa (hen of the woods) as this tends to be greyer than Meripilus giganteus and I think the pores are a bit bigger too, the pores on this were relatively large. The grey colour is correct in this image. I was however worried by the habitat for the fungus, on a fallen birch stem, which is not the usual habitat for either of these fungi.
Flaxton, do you think P. tuberaster with pores as small as this, unfortunately i don't have a picture of the ruler next to pores directly to get a measurement, just my impression was they were small and the other one http://www.ispot.org.uk/node/83868 had very much larger pores.
They are certainly smaller than I have come across in P. tuberaster before (that's one of the reasons for only the "it might be" rating.
Lat/Lng: 50.827625, -1.561856
OS grid ref: SU309032