  The cross-section for prompt antiproton production in collisions of protons
with an energy of $6.5$ TeV incident on helium nuclei at rest is measured with
the LHCb experiment from a data set corresponding to an integrated luminosity
of $0.5\,nb^{-1}$. The target is provided by injecting helium gas into the LHC
beam line at the LHCb interaction point. The reported results, covering
antiproton momenta between $12$ and $110\,\mathrm{GeV/}c$, represent the first
direct determination of the antiproton production cross-section in ${\rm p He}$
collisions, and impact the interpretation of recent results on antiproton
cosmic rays from space-borne experiments.
