  We reconsider the possibility of spontaneous breaking of $R$ parity in the
Minimal Supersymmetric Standard Model. By a renormalization group analysis we
find the parameter space in which a sneutrino gets a vacuum expectation value,
leading to the spontaneous breaking of the lepton number and to the appearance
of a phenomenologically unacceptable massless Goldstone boson. We then analyze
the effect of operators giving rise to a tiny amount of explicit violation of
lepton number, which could emerge as remnants of physics at some superheavy
(Planck or GUT) scale in the low energy effective theory. We show that the
conspiracy between the spontaneous and the explicit breaking scales can provide
a mass to the Goldstone boson larger than the ${\rm Z}^0$ boson mass, hence
allowing for a non vanishing sneutrino vacuum expectation value without
increasing the invisible width of the ${\rm Z}^0$.
