This paper presents a probabilistic analysis method to explicitly include the influences of fire service intervention (FSI) in structural reliability analysis of unsprinklered buildings exposed to fire. The method considers different scenarios of fire behaviour following FSI, based on probabilistic analysis of the current UK fire data to estimate different components of intervention time, size of the fire on arrival of the fire service and extent of damage of the fire at the end of intervention. These statistical data are then used to generate random fire temperature-time curves to calculate the probability of structural failure of intumescent coating protected steel members using the Monte Carlo Simulation method. To aid deterministic design, a simplified method is developed to derive fire load modification factors (δn,FSI) to account for the effects of FSI so that when the fire load modification factor is applied, the deterministic design method gives the same probability of structural failure as that from explicit probabilistic calculation. An extensive parametric study covering a wide range of practical design factors has been carried out and an Analysis of Variance (ANOVA) is performed on the results to examine whether any design factor has significant effect on the fire load modification factor. The results show that fire growth rate and compartment floor area are the most influential. The effects of fire load, steel critical temperature and fire protection thickness have moderate effects. Based on the results of this study, it is suggested that the design fire load should not be reduced if the fire floor area does not exceed 250 m2, or the fire growth rate is ultrafast. If the fire growth rate is fast, the design fire load should not be reduced for fire floor areas less than 1000 m2. For other situations, a fire load modification factor of δn,FSI = 0.78, as in the current Eurocode, may be applied.