Recent trends in market liberalization and increasing congestion in the European interconnected grid have motivated studies of parameters provided to power markets, especially in the intraday and balancing timeframes. In this paper, we present a method using single-box inner polytope approximation to co-optimize the Available Transfer Capacities (ATCs) between multiple bidding zones. The feasible region is bounded by linear flow constraints consisting of AC PTDFs, computed for N−1 contingency cases. Thanks to its formulation as a linear programming problem, the method provides efficient capacity computation, well-suited for applications close to real-time. The method is validated using the IEEE 39-Bus model and the behaviour of the algorithm is demonstrated using real case studies on the Swiss transmission system. Results demonstrate the formulation’s computational efficiency and enable analysis of the linearization accuracy. The proposed method is implemented in a near-real time decision support tool used to compute N−1 secure ATCs, which is in operation in the control room of Swissgrid, the Swiss Transmission System Operator.