Maintaining the structural integrity against severe loading conditions and accidental loads is one of the primary concerns when designing multi-storey modular buildings. Connections between the modular units play a central role in providing integrity in modular buildings. This paper describes the development of an innovative interlocking system for improving the integrity of multi-storey modular buildings. The concept of Modular Integrating System (MIS) and the procedure used to develop an efficient interlocking system, which can be widely used in the construction of modular buildings, is investigated. MIS is a patented joining system including a set of interlocking connections and the method of assembly of modular units that provides a high level of integrity that prevents accidental disassembly and stress concentrations at the points of attachments in case of extreme loading occurrence. The creative easy to install, self-fit and self-locking mechanism of this system can also considerably facilitate the automated assembly of modular buildings and provide an effective solution for controlling construction tolerance. The robustness provided by the proposed system is demonstrated through numerical and experimental analysis.