The basic meanings of legal and lawful, as well as the distinctions in meaning between them, are explained in the entry ILLEGAL. Legal means "according to human law," but lawful has further meanings of "conforming to moral principle" and " in line with religious or ethical doctrine." Thus one refers to "legal rights" or "lawful rights" but is more likely to say "lawful marriage" than "legal marriage" and "lawful living" than "legal living."
Dictionary of problem words and expressions. Harry Shaw. 1975.