A technique is presented to measure the efficiency with which c-jets aremistagged as b-jets (mistagging efficiency) using t (t) over bar events,where one of theW bosons decays into an electron or muon and a neutrinoand the other decays into a quark-antiquark pair. The measurementutilises the relatively large and known W -> cs branching ratio, whichallows ameasurement to be made in an inclusive c-jet sample. The datasample used was collected by the ATLAS detector at root s = 13 TeV andcorresponds to an integrated luminosity of 139 fb(-1). Events arereconstructed using a kinematic likelihood technique which selects themapping between jets and t (t) over bar decay products that yields thehighest likelihood value. The distribution of the b-tagging discriminantfor jets from the hadronic W decays in data is compared with that insimulation to extract the mistagging efficiency as a function of jettransverse momentum. The total uncertainties are in the range 3-17\%.The measurements generally agree with those in simulation but there aresome differences in the region corresponding to the most stringent b-jettagging requirement.