A technique is presented to measure the efficiency with which c-jets are mistagged as b-jets (mistagging efficiency) using Formula omitted events, where one of the W bosons decays into an electron or muon and a neutrino and the other decays into a quark-antiquark pair. The measurement utilises the relatively large and known Formula omitted branching ratio, which allows a measurement to be made in an inclusive c-jet sample. The data sample used was collected by the ATLAS detector at Formula omitted Formula omitted and corresponds to an integrated luminosity of 139 fb Formula omitted. Events are reconstructed using a kinematic likelihood technique which selects the mapping between jets and Formula omitted decay products that yields the highest likelihood value. The distribution of the b-tagging discriminant for jets from the hadronic W decays in data is compared with that in simulation to extract the mistagging efficiency as a function of jet transverse momentum. The total uncertainties are in the range 3-17%. The measurements generally agree with those in simulation but there are some differences in the region corresponding to the most stringent b-jet tagging requirement.