Looks to be possibly a 105mm, or a 155mm (depending on your shoe size) check the brass rotator bands for markings, there may be some info there. Accurate measurements in mm of the width are needed to be sure of an id, and a length will help also. As was already stated, it's not fused, and therefore not dangerous to handle. While there may be explosive inside, all you need to do is wet it down and flush it out, looks like it had been there a while though so there's probably nothing but mud inside. Don't call the law if you want to keep it or ever see it again, they'll "destroy" it, which really means there's a good chance it'll find it's way to someone elses collection seeing as it isn't a live round.
They're are a lot of ordinance collectors out there and if you decide to sell it you'll have no problem finding someone interested. Where did you come across it?