The present study considers a unionised (nonlinear) duopoly with two different labour market institutions, i.e. efficient bargaining (EB) and right to manage (RTM), to analyse product market stability under quantity competition with trade unions. We show that when the preference of unions towards wages is small, (i) the parametric stability region under RTM is higher than under EB, and (ii) a rise in the union power in the Nash bargaining played between firms and unions monotonically increases (resp. reduces) the parametric stability region under RTM (resp. EB). In contrast, when the preference of unions becomes larger, an increase in the union's bargaining power acts: (1) as an economic stabiliser when the union power is small; (2) as an e...