"Acapella"

Hello. I'm a new poster on this grop.
I've got a question - can anyone tell me, what's the proper spelling of "a capella" in English? Would be good in AmEng and BrEng, if it differs. Why I'm asking? I've seen many spellings - "a capella", "acapella", "acappella", etc. etc. - and I have no idea which is the correct one. Is there any rule, or is it - while being "imported" from Italian - still in the process of "Englishization" and there is no proper spelling strictly defined? TIA for any kind of help.

regards,
CoosteR