  Neutrinos are among the most mysterious particles in nature. Their mass
hierarchy and oscillations, as well as their antiparticle properties, are being
intensively studied in experiments around the world. Moreover, in many models
of physics beyond the Standard Model, the baryon asymmetry or the dark matter
density in the Universe are explained by introducing new species of neutrinos.
Among others, heavy neutrinos of Dirac or Majorana nature were proposed to
solve open questions in High Energy Physics. Such neutrinos with masses above
the EW scale could be produced at future linear e$^+$e$^-$ colliders, like the
Compact LInear Collider (CLIC) or the International Linear Collider (ILC).
  We studied the possibility of observing decays of heavy Dirac and Majorana
neutrinos in the $qq\ell$ final state with ILC running at 500 GeV and 1 TeV,
and CLIC at 3 TeV. The analysis is based on the Whizard event generation and
fast simulation of detector response with Delphes. Neutrinos with masses from
200 GeV to 3.2 TeV were considered. We estimated the limits on the production
cross sections, interpreted them in terms of the neutrino-lepton coupling
parameter $V_{\ell N}^{2}$ (effectively the neutrino mixing angle) and compared
them with current limits coming from the LHC running at 13 TeV, as well as the
expected limits from future hadron colliders. The limits for the future lepton
colliders, extending down to the coupling values of $10^{-7} - 10^{-6}$, are
stricter than any other limit estimates published so far.
