Much of the 400 million litres of water estimated to leak from UK toilets every day can be blamed on dual-flush toilets according to a radio programme due to air on BBC Radio 4.

The environmental programme, Costing the Earth, to be aired later today (29 September), has talked to a number of organisations claiming that dual-flush toilets, which are supposed to help people save water, are in fact highly wasteful. 

Andrew Tucker, water efficiency manager at Thames Water, the UK's largest water and sewerage company, says the problem is getting worse as more and more bathrooms are upgraded.

"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 issue is reportedly due to a combination of leaks and confusing design. 

In 2001 a change in regulation allowed the use of drop valve systems, which are now used by most dual-flush toilets.

However those systems, because they sit at the bottom of the cistern and can get clogged up with debris, are more prone to leakage than the traditional siphon, something confirmed by the Bathroom Manufacturers Association (BMA).

Waterwise, an NGO dedicated to saving water, estimates that 5% to 8% of toilets are leaking and says most of these are dual-flush.   

Now bathroom manufacturers are being asked to "design the problem out of existence" but BMA Chief Executive Tom Reynolds does not want dual-flush toilets to be banned: "Individual companies have gone into looking at the issue… but we haven't cracked what the underlying issue is. 

"There is a commercial and moral imperative to ensure our products reduce leakage wherever possible."

Further wastage appears to be caused by some confusion over which buttons to use when flushing. 

A recent survey by Thames Water shows that as many as 50% of customers chose the wrong button - or pushed both.

-> The Great Leaky Loo Scandal, Costing the Earth, BBC Radio 4