Offices are needlessly wasting 310 million litres of water every day, a business advice group has warned.
Envirowise, said it was happening while householders across Britain were being urged to save water during the worst drought for more than a decade.
In a bid to make businesses play their part in the battle against water shortages, Envirowise is launching "Water at Work Week" today and asking "Have you got the bottle?" to save water in one of the simplest ways possible - putting a litre bottle of water in older toilet cisterns.
Figures from the programme which works with firms to help combat resource waste, shows that office work-ers use as much as 35 litres of water every day, 86 per cent of which is a direct result of flushing toilets.
With more than 22 million office workers throughout the UK, this alone equates to 663 million litres a day or enough to fill the Royal Albert Hall nearly seven times.
However, if every office were to put bottles in their toilets' cisterns, daily water wastage could be reduced by a fifth - the equivalent of 35 Olympic swimming pools.
By installing push or spray taps in washrooms, water wasted from hand washing could also be reduced by a further 20 million litres.
To demonstrate the vast quantity of water wasted by businesses, Envirowise has created an underwater office in a tank at the London Aquarium.