Inclusive jet differential cross sections for the reaction e + p → e + + jet + X with quasi-real photons have been measured with the ZEUS detector at HERA. These cross sections are given for the photon-proton centre-of-mass energy interval 134 < W < 277 GeV and jet pseudorapidity in the range −1 < η jet < 2 in the laboratory frame. The results are presented for three cone radii in the η − ϕ plane, R = 1.0, 0.7 and 0.5. Measurements of dσ/dη jet above various jet-transverse-energy thresholds up

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... o 25 GeV and in three ranges of W are presented and compared to next-to-leading order (NLO) QCD calculations. For jets defined with R = 1.0 differences between data and NLO calculations are seen at high η jet and low E jet T . The measured cross sections for jets defined with R = 0.7 are well described by the calculations in the entire measured range of η jet and E jet T . The inclusive jet cross section for E jet T > 21 GeV is consistent with an approximately linear variation with the cone radius R in the range between 0.5 and 1.0, and with NLO calculations. The ZEUS Collaboration: High-ET inclusive jet cross sections in photoproduction at HERA