The flexural behaviour of sandwich beam panels with integrated reinforcement and expanded polystyrene (EPS) core is evaluated at different shear span to depth (a/d) ratios. The sandwich beam is a lightweight, thin member consisting of two reinforced concrete structural wythes with diagonal truss-type connectors. In this experimental study, the strain profiles are measured accurately using digital image correlation (DIC) to study the level of composite action in flexure. The measured strain profiles indicate a significant level of independent bending in the two wythes that are accommodated within the shear deformation of the core. The cracking moment of the beam panels depends on the a/d ratio. The stresses obtained from sectional analysis and composite beam theory do not account for the shear deformation of the EPS core and significantly over-estimate the initial stiffness and cracking load. Flexural response predicted by sandwich beam theory considering an anti-plane core is derived and used to estimate the stress and strain states in the beam panel cross-section. The sandwich beam theory gives a correct representation of the experimentally measured strain profile in flexure and an accurate estimate of the cracking load. The dependence of the flexural cracking moment on the a/d ratio is also accurately predicted. The displacement profiles from DIC indicate that the flexural cracking in the sandwich panel produces a hinge-type opening of the crack. The progressive opening of the hinge produces a localized flexural failure, and the conventional sectional analysis derived from composite beam theory gives a conservative prediction of the ultimate flexural capacity. © 2021 Institution of Structural Engineers