Do I need to register as self-employed if I am already employed with PAYE and receive separate royalties?

I am employed on the normal PAYE system at the normal rate. I am about to start receiving monthly royalty payments which are not insubstantial. Do I need to register as self-employed, incurring NIC cles 2 and 4, or can I just clify the royalties as some sort of extra income and only pay income tax on them?