Aims: We report the first detection of the J = 1-0 (102.6 GHz) rotational lines of CF+ (fluoromethylidynium ion) towards CygX-N63, a young and massive protostar of the Cygnus X region. Methods: This detection occurred as part of an unbiased spectral survey of this object in the 0.8-3 mm range, performed with the IRAM 30 m telescope. The data were analyzed using a local thermodynamical equilibrium model (LTE model) and a population diagram in order to derive the column density. Results: The line velocity (-4 km s-1) and line width (1.6 km s-1) indicate an origin from the collapsing envelope of the protostar. We obtain a CF+ column density of 4 × 1011 cm-2. The CF+ ion is thought to be a good tracer for C+ and assuming a ratio of 10-6 for CF+/C+, we derive a total number of C+ of 1.2 × 1053 within the beam. There is no evidence of carbon ionization caused by an exterior source of UV photons suggesting that the protostar itself is the source of ionization. Ionization from the protostellar photosphere is not efficient enough. In contrast, X-ray ionization from the accretion shock(s) and UV ionization from outflow shocks could provide a large enough ionizing power to explain our CF+ detection. Conclusions: Surprisingly, CF+ has been detected towards a cold, massive protostar with no sign of an external photon dissociation region (PDR), which means that the only possibility is the existence of a significant inner source of C+. This is an important result that opens interesting perspectives to study the early development of ionized regions and to approach the issue of the evolution of the inner regions of collapsing envelopes of massive protostars. The existence of high energy radiations early in the evolution of massive protostars also has important implications for chemical evolution of dense collapsing gas and could trigger peculiar chemistry and early formation of a hot core.Appendices are available in electronic form at http://www.aanda.org< Réduire