Detection of the gamma-ray sources with the angular extend of the order of one degree or larger represents a significant challenge for the imaging Cherenkov atmospheric telescopes (IACTs) due to the lack of reliable estimation of the background cosmic rays (CRs) in the telescope field of view of a few degrees. We present a novel method, which allows estimation of the CR background without the prior assumption about the morphology of the gamma-ray emission from a putative source with large angular extension. This novel method, morphology-independent background estimation (MIBE), utilises the "cosmic-ray-like" events (those that fail gamma-hadron-separation cuts based on shower-shape parameters) collected from the given field to estimate the "gamma-ray-like" background in the same field. The method is shown to reduce the systematic error of the background estimate comparing to the conventional approaches for the analysis of gamma-ray emission from sources with large angular extension and with unknown morphology. This submission explains the methodology and presents the validation of the MIBE using the gamma-ray-free data obtained by the VERITAS (Very Energetic Radiation Imaging Telescope Array System).