Assessment of trends in hydrological extremes using regional magnification factors

•A new method for estimating a regional regression slope parameter across a region, or pooling group, of catchment considered hydrologically similar, and where annual maximum events at different sites are cross-correlated.•Application of the new method to a national dataset of annual maximum series from the UK shows that the regional slope estimates are significantly positive (p < 0.05) consistently in the west and north of the country, while mostly not significant in the east and south.•The range of trend estimates translates into a corresponding increase in design flood (as measured by regional magnification factors) of up-to 50% for time horizon of 50-years into the future.