This study on lightweight built-up composite panel comprises of cold-formed profiled steel sheet (CFPSS) built-up with a hot-rolled steel plate (HRSP) connected by bolts. The space between the CFPSS and HRSP was filled with cement mortar as an infill material to study the effect of the composite system's strength and failure behaviour. The testing was carried with eighteen medium scale composite panels under three-point bending tests in two phases. Phase I specimens was fabricated from a single CFPSS and phase II specimens using double (nested) CFPSS, at the bottom. Other parameters include the length of the specimen (700 mm, 1200 mm and 1700 mm) and bolt spacing along the length (L/4, L/6 and L/8). The experimental data were analysed based on idealised load-deflection plots. The results indicate that the shorter and longer length mortar filled specimens experienced higher ultimate load and initial stiffness than the without mortar infill specimens in both the phases. However, infilled mortar specimens in moderate length did not show considerable improvement in ultimate load and initial stiffness values in both the phases. The phase II specimens experienced higher load and ductility in post-debonding failure regime compared to the phase I specimens. Most of the specimens exhibited tearing of CFPSS as a mode of failure at ultimate load. The modified Newmark's analytical expressions using equivalent area method for infilled mortar were derived up to the elastic limit to determine the deflection. The analytical results are in good agreement with the experimental results for both the phases. © 2019 Institution of Structural Engineers