Floods are the most common and widely distributed natural risk, causing over £1Bn of 12 damage per year in the UK as a result of recent events. Climatic projections predict an increase in 13 flood risk; it becomes urgent to assess climate change impact on extreme flows, and evaluate 14 uncertainties related to these projections. This paper aims to assess the changes in extreme runoff 15 for the 1:100 year return period across Great Britain as a result of climate change using the Future 16 Flows Hydrology database. The Generalised Extreme Value (GEV) and Generalised Pareto (GP) 17 models are automatically fitted for 11 ensemble member flow series available for the baseline and 18 the 2080s. The analysis evaluates the uncertainty related to the Extreme Value (EV) and climate 19 model parameters. Results suggest that GP and GEV give similar runoff estimates and uncertainties. 20 From the baseline to the 2080s, increasing estimate and uncertainties is evident in east England. 21 With the GEV the uncertainty attributed to the climate model parameters is greater than for the GP 22 (around 60% and 40% of the total uncertainty respectively). This shows that when fitting both EV 23 models, the uncertainty related to their parameters has to be accounted for to assess extreme runoff.