Between 2004 and 2005, British water companies lost 3,609Ml/day or 3.6Mm3/day, equivalent to 1,444 Olympic swimming pools full of water.
Yet despite the massive volumes that escape daily from our infrastructure network most water companies are meeting their regulatory leak reduction targets (see graph).
'In broad terms we expect companies to contain leakage at the level at which it would cost more to make further reductions than to produce the water from another source.
This is known as the economic level of leakage (ELL), ' says regulator Ofwat chairman Philip Fletcher.
But there is dispute over how this gure is calculated. The Environment Agency, for one, has reservations.
'We have looked at economic level of leakage up to 2030 to track what is happening.
Between 1995 and 2000 it declined but there was an increase between 2000 and 2005, ' says Environment Agency head of water resources Ian Barker.
'It will decline between now and 2010 but it only brings us back to the 2000 levels.
Beyond 2010 it at-lines. I find it difcult to believe that this will remain the economic level despite the pressures on resources from climate change and growth, ' he says.
'We are challenging industry to reconsider how social and environmental costs are considered in the calculations for economic level of leakage.
There is very little evidence that they do this adequately.
'We have to ask: 'are we balancing up everything?' We don't want to look back in 20 to 25 years and be seen as a generation that failed to tackle the problem, ' he says.
One thing everyone seems to agree on is that a target based system where levels are set at an arbitrary reduction level - for example 5% per annum - would not work.
'It is all about balance and we think that the economic level of leakage is the right framework.
The vast majority of companies are performing well. We are not in favour of arbitrary edicts, ' says Ofwat's head of capital maintenance George Day.
'There is debate going on about how leakage in terms of sustainability can be dealt with but we have to accept that water will leak no matter what the age of the main is, ' says Water UK policy advisor Steve Entifo.
'Companies can reduce leakage further by replacing infrastructure but at a cost that comes through to customers.
Do they want that? We also have to consider the environmental impacts of the increased construction activity that more replacement involves and how sustainable that is, ' he says.
Since the passing of the 2003 Water Act, the regulator has had to power to ne water companies which fail to meet their regulatory obligations. But this has not yet been enforced, despite United Utilities seeing leakage rise for the third year running.
The impending threat of nes is helping keep water company eyes on the ball. They plan to carry out mains replacement to reduce leakage by 8%, to 3,335Ml/day by 2010 - the equivalent of 1,334 Olympic sized swimming pools, just 110 fewer.