by Chirag Trivedi
Are you one of those diners who feels just a little embarrassed about asking for it?
Is tap water better than bottled?
After all, not only might your request attract the snooty disdain of a waiter, fellow customers might take you for a skinflint.
But fear not - from now on, your demand to be given tap water instead of bottled in a restaurant or bar has some weighty political backing.
In fact, the campaign group London on Tap and Mayor of London Ken Livingstone are urging people to shun pricey packaged water while dining out, in favour of asking for a jug.
However, the British Hospitality Association warned restaurateurs may raise prices of other products to compensate for any loss of profits.
According to Thames Water, 70% of people say the price of mineral water in restaurants is too expensive and one in five people are "slightly nervous" or "too scared to ask" for tap water.
Thames Water chief executive David Owens said tap water was 500 times cheaper than bottled water.
He added: "We all know the benefits of keeping hydrated and luckily, in London, we have probably the best drinking water in the world.
"We should all be proud to drink it."
And Jenny Jones, Green Party member on the London Assembly, said: "This is a simple, money-saving way to brush up your eco-credentials. Say "no" to bottled water and help save the planet."
Dame Yve Buckland, chairwoman of the Consumer Council for Water group, said: "The bottled water industry spends millions investing in their brands and that's what people are paying for when they pick up a bottle of water.
"There is no health advantage in drinking bottled water."
London On Tap says imported bottled water produces up to 300 times more carbon dioxide per litre than tap water.
But Jo Jacobius, British Bottled Water organisation, said: "People seem to forget the collecting, extraction and cleansing of mains water also produces emissions.
"Thames water leaks seven million litres of water a day in London alone - how much energy is being wasted there?
"At the end of the day, it is also about personal choice. People can choose water that has been naturally cleaned or cleaned artificially."
And Martin Crouchman, of the British Hospitality Association, said there was no legal requirement on bars and restaurants to provide tap water - although he knew of none which would refuse to provide it.
He added if a restaurateur sees his profits fall, as sales of bottled water drop, then he may consider raising other prices.
"They will have to take a commercial decision," he said. "They may have to raise the mark-ups on food or other items."
But one restaurant in London is actively giving away tap water.
The Waterside is Shoreditch has filtered water pumped around the premises in a network of pipes, which also cools the restaurant.
Owner Arthur Potts-Dawson said: "There tends to be a lot of water snobbery in the restaurant industry.
"So what we are doing here is giving away free tap water - as it should be".
Last year Claridges in central London introduced what is thought to be the world's most expensive bottled water.
The 420 Volcanic, from New Zealand, sells at £21 for 42cl or about £50 a litre.
But a recent survey found people may be wasting their money on the bottled variety.
A taste test comparing bottled water brands with tap water was carried out by Decanter magazine last December.
Thames Water's tap water was rated third out of 24 varieties tested.
Part of the London on Tap campaign, which has been organised by Thames Water and the Mayor of London, will be a competition, open to all London-based designers, to design a carafe made from recycled glass to be used across the capital.