Billions of litres of water are being wasted every week because of an innovation intended to cut water use.
About 400 million litres 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.