The spelling "percent" (one word) is generally accepted, I think. Thus I presumed that "permille" (one word) would be correct, but the Cambridge Dictionary only mentions "per mille". On the other hand, the MS Word spell check only accepts "permille" (the same is the case for the spell check in the editor on this site).
Google search seem to favor "per mille" and Wikipedia also uses that spelling, but mentions "permille" as an alternative.
So which is more correct? (and does there exist a good authoritative source apart from the SE users?)