  The analysing power of the reaction ${\rm n}{\rm p} \to {\rm p}{\rm p}
\pi^{-}$ for neutron energies between threshold and 570 MeV has been determined
using a transversely polarised neutron beam at PSI. The reaction has been
studied in a kinematically complete measurement using a time-of-flight
spectrometer with large acceptance. Analysing powers have been determined as a
function of the c.m. pion angle in different regions of the proton-proton
invariant mass. They are compared to other data from the reactions ${\rm n}{\rm
p} \to {\rm p}{\rm p} \pi^{-}$ and ${{\rm p}{\rm p} \to {\rm p}{\rm p}
\pi^{0}}$. The np elastic scattering analysing power was determined as a
by-product of the measurements.
