The success rate of construction projects is generally low, with the majority failing to meet the established success criteria and a significant number of projects experiencing cost and time overruns. In this research study, factors contributing to time and cost overruns in public construction projects were identified through a comprehensive literature review. Data was collected through questionnaires, and the Relative Importance Index (RII) method was used to rank these factors. The top four most critical factors were the client's financial issues, the selection of incompetent and inexperienced contractors, payment delays by the client to contractors, and the contractor's financial problems, with RII values of 0.831, 0.819, 0.805, and 0.794, respectively. After ranking the critical factors through the RII method, the Interpretive Structural Modelling (ISM) technique was employed to analyze the interrelationships among these factors, highlighting their mutual influences and hierarchical levels. The results of the ISM suggested that the client’s financial difficulties and the selection of an inexperienced contractor were positioned at level 4 of the hierarchy, highlighting their critical importance. The results of the RII and ISM methods aligned, with both identifying the financial difficulties of the client and the selection of an inexperienced contractor as the two most critical factors ranked highest by the RII method and positioned at the top of the ISM hierarchy. The Identification of critical factors of delay and cost overrun will help decision-makers mitigate their impact by implementing preventive measures and allocating maximum attention and resources to address them.