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In recent years, UK summer heatwaves have resulted in thousands of excess deaths, with both extreme temperatures and high humidity increasing health risks. Here, the UK Climate Projections 2018 (UKCP18) are compared to observational (HadUK-Grid) and reanalysis data (ERA5) to quantify model performance at capturing mean, extremes (95th to 99.5 th percentiles) and variability in the climate state and heat stress metrics (simplified wet bulb global temperature, sWBGT; Humidex; apparent temperature). Simulations carried out for UKCP18 generally perform as well as or better than CMIP5 models in reproducing observed spatial patterns of UK climate relating to extreme heat, with RMSE values on average∼ 30% less than for the CMIP5 models. Increasing spatial resolution in UKCP18 simulations is shown to yield a minor improvement in model performance (RMSE values on average∼ 5% less) compared …