“Instead, alongside constructing and maintaining defences, we must help communities learn to live with risk, minimise damage, and return to normal life quickly,” it said.
The report warned that there is too much focus on trying to stop climate change and not enough on preparing for its effects, including flooding, coastal erosion and depleted drinking water supplies.
Population growth and climate change will also put more pressure on the water system, the agency said.
If no additional measures are introduced between 2025 and 2050, more than 3.4 billion extra litres of water per day will be needed to ensure public water supplies, its figures show.
Necessary improvements include restoring peat bogs and wetlands to improve water quality and availability, which is under pressure from growing demand, pollution and physical changes to rivers and the coast.
Sewer systems which allow raw sewage to be dumped in rivers and the sea will also have to be used more frequently, the agency said, causing more pollution.