The composition of ices in comets may reflect that of the molecular cloud i
n which the Sun formed, or it may show evidence of chemical processing in t
he pre-planetary accretion disk around the proto-Sun. As carbon monoxide (C
O) is ubiquitous in molecular clouds(1,2), its abundance with respect to wa
ter could help to determine the degree to which pre-cometary material was p
rocessed, although variations in CO abundance may also be influenced by the
distance from the Sun at which comets formed(3-5). Observations have not h
itherto provided an unambiguous measure of CO in the cometary ice (native C
O). Evidence for an extended source of CO associated with comet Halley was
provided by the Giotto spacecraft(6-9), but alternative interpretations exi
st(10). Here we report observations of comet Hale-Bopp which show that abou
t half of the CO in the comet comes directly from ice stored in the nucleus
. The abundance of this CO with respect to water (12 per cent) is smaller t
han in quiescent regions of molecular clouds, but is consistent with that m
easured in proto-stellar envelopes(11), suggesting that the ices underwent
some processing before their inclusion into Hale-Bopp. The remaining CO ari
ses in the coma, probably through thermal destruction of more complex molec
ules.