  $^{1}S_{0}$ \textit{nn}-scattering length has been measured in the
\textit{nd}-breakup reaction at 60 MeV. The experiment was performed in the
final state interaction geometry with registration of all three secondary
particles. The scattering length $a_{nn} = -16.3 \pm 0.5$~fm was obtained from
a comparison of the experimental dependence of the \textit{nd}-breakup yield on
the relative energy of \textit{nn}-pair with the simulation results. An
analysis of this value of scattering length together with the data obtained in
other experiments on \textit{nd}- and \textit{dd}-breakup confirms the
hypothesis about the influence of $3N$-forces on the values of
\textit{nn}-interaction parameters extracted in these reactions.
