You will have to grind the mushrooming off to see how far the crack has traveled.
The mushrooming has to be removed anyway as it could chip off - bad for the eyes
Hopefully it's only cracked in the mushrooming which is very common when pushed to those limits
Alan
By failing to prepare, you are preparing to fail.(Benjamin Franklin)
It's an old Kent pattern axe, they were mass-produced usually from good steel and tend to have a slim profile for the most part. Ideal for cutting rather than splitting, but only a top down look can confirm that for sure.
You could grind the crack off if you wanted, or at least back a bit, but unless you are belting the hell out of it you are unlikely to make it any worse. Sharpened up, it should be fine. They make fine green woodworking tools.
Thanks for the advice guys