Irish Water had warned that it suspected as much as half the supply was being lost, but figures previously collected included partial estimates and put the average loss per county at 39.5%.

Roscommon has the biggest problem, losing 68% of its water through leaks — a figure that is 6% higher than previously thought.

Ten other councils are losing at least half their supply to leaks. They are: Mayo, 58%; Cork county, 57%; Kerry, 57%, Galway city, 56%; Offaly, 56%; Leitrim, 55%; Sligo, 54%; Waterford city and county, 52%; Galway county, 52%; and Dun Laoghaire Rathdown, 50%.

Losses were lowest in Monaghan, 32%; Cavan, 33%; South Dublin, 34%; Kildare, 35%; Laois, 35%; and Wicklow and Clare, both 37%.

The figures, collated by the National Oversight and Audit Commission (NOAC), are correct up to the middle of 2015 and are considered the most reliable yet because they use data from domestic water meters.

Previously, only commercial water use was metered so household consumption was estimated and subtracted from the known quantities of water supplied.

“The rollout of domestic metering by Irish Water across the county in 2014 has enabled a calculation of UFW [unaccounted for water] using data based on actual domestic consumption from Irish Water which now presents a more robust baseline,” said NOAC.

Some 50m litres of water are lost every day and the scale of repair and replacement works required to stop it are so vast that Irish Water says it can at best hope to reduce the loss to 38% by 2021 and to 20% by 2040.

The water lost was of very high quality — figures show that 99.4% of public drinking water supplies were in compliance with quality standards and two local authorities, Longford and Louth, scored 100%.

Private drinking water supplies had a slightly lower compliance rate of 97.9% with just one area, Dún Laoghaire Rathdown, being 100% compliant.