We propose a novel test to assess the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, VCKM, at present and future collider experiments. Our strategy makes use of the W+W− production cross section to directly probe the VCKM†VCKM product, which regulates the high-energy behavior of the observable. The violation of unitarity is signaled by an anomalous behavior of the cross section that grows quadratically with the W+W− invariant mass with respect to the Standard Model prediction. By using the recent ATLAS measurements of the W+W− cross section we are able to constrain the maximal unitarity violation allowed by current data, producing a bound complementary to the results of flavor physics experiments. Forecasts for the high luminosity phase of the LHC and for the future 100 TeV hadron collider are also discussed.
