Legacy phosphorus (P) from lake sediment impedes catchment based measures aimed at improving water quality. While evidence based water quality recovery targets are essential for sustainable lake management there is little information on the long-term behaviour of internal reservoirs of P. We used steady state empirical lake models to characterise the long-term changes in internal loading of P against a backdrop of changing lake temperature and catchment inputs. Phosphorus mass balances for five decades were used to compare scenarios of internal loading and its changing influence on lake water P in Lough Neagh, the largest lake in Ireland. Internal P loading increased rapidly in the mid 1990s, at a rate of 43.5 mg m-2 yr-1 and remained elevated. Annual mean lake TP was typically over 30 μg L-1 greater in the second half of the timeseries as external P loading remained elevated above critical levels for recovery. Annual mean net internal loading of P increased from 243 to 577 mg TP m-2 yr-1, equivalent to an increase from 25 % to 47 % of annual mean lake TP concentration. Lake water temperature, external P loading, and pH respectively were the most influential drivers respectively of sediment P release. Negative impacts on water quality will persist for decades as the greater mass of recycled legacy P continues to prevent the achievement of water targets in the short to medium term.