Billions of litres of water are being wasted every week because of an innovation intended to cut water use.
About 400 million litres (88 million gallons) of water are estimated to leak from UK toilets every day.
But much of this can be blamed on dual-flush toilets, designed to save water, the BBC has learned.
One water company says dual-flush toilets are now wasting more water than they save due to a combination of leaky mechanisms and confusing flush buttons.
Dual-flush toilets allow the option of a small or large flush - typically four or six litres - to only use the water necessary for urine or faeces.
Water-saving organisation Waterwise estimates that between 5% and 8% of toilets are leaking and says most of these are dual flush.
Andrew Tucker, water efficiency manager at Thames Water, the UK's largest water and sewerage company, says the problem is getting worse as bathrooms are done up.
"Because we've got so many [loos] that continuously flow all through the day, collectively that water loss is now exceeding the amount of water they should be saving nationally," he said.
"The volume of water loss is getting bigger every day as more people refurbish and retrofit their older toilets and as we build more homes, so we're actually adding a problem."
The 400 million litres (88 million gallons) of water lost daily from loos is enough to supply 2.8 million people - the populations of Edinburgh, Cardiff, Belfast, Manchester, Sheffield, Liverpool and Bristol combined.
Rising demand for water harms the natural environment and has been strongly linked to rivers running dry.