The field K((G)) of generalized power series with coefficients in the field K of characteristic 0 and exponents in the ordered additive abelian group G plays an important role in the study of real closed fields. Conway and Gonshor (see [2, 4]) considered the problem of existence of non-standard irreducible (respectively prime) elements in the huge "ring" of omnific integers, which is indeed equivalent to the existence of irreducible (respectively prime) elements in the ring K((G$^{\leq 0}$)) of series with non-positive exponents. Berarducci (see [1]) proved that K((G$^{\leq 0}$)) does have irreducible elements, but it remained open whether the irreducibles are prime i.e.; generate a prime ideal. In this paper we prove that K((G$^{\leq 0}$)) does have prime elements if G = ($\mathbb{R}$, +) is the additive group of the reals, or more generally if G contains a maximal proper convex subgroup.