An experimental and numerical investigation was conducted to characterise the influence of the cellulosic substrate used to assess the ignition propensity (IP) of cigarettes. The objective was to better understand the poor repeatability of the test and eventually propose alternative substrates. It is believed that such a low test reproducibility is mainly due to the variability of the cigarette itself but it is unclear as to what extent the substrate may also influence the variability of the test results. An insight to this question was obtained by first characterising the thermodynamic behaviour of the substrate experimentally and then constructing a numerical model that quantified the relative influence of the distinct parameters. A suite of experimental methods that included TGA, DSC, infrared measurements and laser triangulation among others was used to measure the thermophysical properties of the substrate. These properties were then used to build a CFD model that simulated the smouldering combustion experienced by the substrate during the IP test. After validating the model against contactless temperature measurements, a parametric study consisting of 363 IP simulations was performed, which served to quantify the relative importance of each influencing parameter. The results indicated that the heat capacity, pyrolysis activation energy, and air gap thickness are the most influencing aspects of the substrate. The latter parameter was found to significantly vary within each test and its influence was comparable to that of some major properties of the cigarette such as the cigarette's temperature and burning rate. It is therefore postulated that the variability of the substrate itself plays an important role on the poor repeatability of the test and it may comprise its reliability. A software called SIMULIP-Software was developed to facilitate the calculation of effect of the substrate in the IP testing and could be used for future developments.