Mike,
You asked......."Authorised by whom precisely?"
I seem to remember that the Flight Manual forms part of an aircraft's Certificate of Airworthiness. The Flight Manual contains the manufacturer's Normal and Emergency Procedures for operating the aircraft. Of course, we are legally obliged to operate the aircraft in accordance with its CofA and hence its Flight Manual. Therefore, strictly speaking, we're obliged to use the checklist in the Flight Manual (not the Pilots Operating Handbook, since it's the Flight Manual. not the POH, that's kept up to date - manufacturer's and/or CAA amendments, etc.).
So, there's the answer that the authorities might give you.
Now, I'm not saying that there's no place for third-party checklists, far from it. In fact that's exactly what I've always used and they've been perfectly adequate in my personal view. But you did ask the question.