As has been mentioned, it's a fairly common occurrence when the pinch bolts are over-tightened. There are lots of cases detailed in threads on the form on Rs, Xs and Zs. Here's just one:

http://www.xt660.com/showthread.php?t=13307&highlight=fork+cracked

I wouldn't risk riding it in that state. It certainly won't pass (or shouldn't pass an MOT) in that condition. Now you've advertised that you are aware of the fault on an open forum, your insurance company (should they get wind of it) is highly unlikely to pay out in an accident.

You can get this sort of thing repaired professionally - it is most definitely NOT a DIY job! Sourcing a pair of secondhand forks is probably the best way forward.

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