A measurement of C P violation in the decay B + → K + π 0 is reported using data corresponding to an integrated luminosity of 5.4 fb − 1 collected with the LHCb experiment at a center-of-mass energy of √ s = 13 TeV . The C P asymmetry is measured to be 0.025 ± 0.015 ± 0.006 ± 0.003 , where the uncertainties are statistical, systematic, and due to an external input. This is the most precise measurement of this quantity. It confirms and significantly enhances the observed anomalous difference between the direct C P asymmetries of the B 0 → K + π − and B + → K + π 0 decays, known as the K π puzzle.