To me it looks like a tooth shaped flint, but it is difficult to tell from a photo. If it were a tooth it would be something more like a shark tooth rather than a T-Rex tooth judging by the shape of it.
Not a fossil tooth I'm afraid Timothy! I agree with Rhys, it looks like it could be a flint nodule, which are rocks often mistaken for fossil teeth and other kinds of fossils too. But it is hard to be sure from the picture is it is flint.
As Rhys says, the triangular shape is more like a sharks tooth, but it is not one I'm afraid, as the shape is too irregular and the texture wrong.
These websites have some good tips for finding fossils: