Abstract The interpretation of cosmic antiproton flux measurements from space-borne experiments is currently limited by the knowledge of the antiproton production cross-section in collisions between primary cosmic rays and the interstellar medium. Using collisions of protons with an energy of 6.5 $$\,\text {Te\hspace{-1.00006pt}V}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mspace /> <mml:mtext>Te</mml:mtext> <mml:mspace /> <mml:mtext>V</mml:mtext> </mml:mrow> </mml:math> incident on helium nuclei at rest in the proximity of the interaction region of the LHCb experiment, the ratio of antiprotons originating from antihyperon decays to prompt production is measured for antiproton momenta between 12 and $$110\,\text {Ge\hspace{-1.00006pt}V\!/}c $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>110</mml:mn> <mml:mspace /> <mml:mtext>Ge</mml:mtext> <mml:mspace /> <mml:mtext>V\!/</mml:mtext> <mml:mi>c</mml:mi> </mml:mrow> </mml:math> . The dominant antihyperon contribution, namely $${\overline{\varLambda }} \rightarrow {\overline{{p}}} {{\pi } ^+} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mover> <mml:mi>Λ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mo>→</mml:mo> <mml:mover> <mml:mi>p</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:msup> <mml:mrow> <mml:mi>π</mml:mi> </mml:mrow> <mml:mo>+</mml:mo> </mml:msup> </mml:mrow> </mml:math> decays from promptly produced $$\overline{\varLambda }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mi>Λ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> particles, is also exclusively measured. The results complement the measurement of prompt antiproton production obtained from the same data sample. At the energy scale of this measurement, the antihyperon contributions to antiproton production are observed to be significantly larger than predictions of commonly used hadronic production models.