Two weeks ago Dr Anne Guilding and her friends were relaxing in a restaurant, celebrating her son's wedding the day before. The food, wine and conversation were flowing. But then it suddenly turned ugly. No, there wasn't a bust-up between the in-laws. It was because one of the diners dared ask for some tap water so she could take a tablet.

"We had already spent hundreds of pounds, buying wine and expensive sparkling water. So I was gobsmacked when the waiter refused to bring us a jug of tap water, or even a glass. He told us it wasn't the restaurant's policy to serve tap water," says Guilding, whose party at the Brasserie El Prado in Bridgend included a member of the Welsh Assembly and a visiting American astonished that British restaurants don't automatically serve free water.

"I asked to speak to the manager, and explained that we had bought plenty of drinks and wanted some normal water. One member of the party needed to take medication and we did not consider asking for a jug of water unreasonable. He flatly refused to let us have any, reiterating that it was not the restaurant's policy."

Guilding left the restaurant angry, embarrassed and humiliated. But ultimately the restaurant was the loser - the group decided not to leave a tip. The restaurant's owner, Jesus Decelas, says: "Water costs us, the ice costs us, the lemon costs us. I have to charge."

According to the National Consumer Council, Guilding's experience is depressingly common in Britain. Its research found that nine out of 10 restaurants fail to offer free tap water.

A look at the Consumer Council for Water's website, where restaurant-goers can post reports of restaurant behaviour, reveals widespread anger and indignation about charging for tap water.

One poster, "Sharon", says: "When I was in Burger King recently, I started coughing and asked for some tap water. They refused even though I had tears running down my cheeks, as I struggled to stop coughing. They did offer to sell me a bottle of water; however, I decided to buy a coke instead as that was cheaper. Apparently, they don't 'do' tap water!" Burger King told Guardian Money that the restaurant was wrong. It is company policy to provide customers with tap water on request.

At a wedding in Birmingham, the hotel demanded £5 per bottle of water placed on tables, and resisted fiercely when the couple asked for jugs of tap water instead. Pubs are named and shamed for charging as much as £1 for a glass of tap water. Airports and railway stations are regularly cited as money-grabbers, removing free water fountains and effectively forcing travellers to buy bottled water at food outlets.

Some restaurants even tell customers that water can't be provided for "health and safety" reasons.

Dame Yve Buckland, chair of the Consumer Council for Water, says: "As a country we have put £70bn into the water industry. Tap water in Britain is now of the very highest quality and we urge restaurants to offer tap water to customers for free. You shouldn't have to feel like a cheapskate because you ask for tap water."

But can you force a restaurant to give you free tap water? The tales on CCW's website (ccwater.org.uk) reveal widespread confusion over consumer law.

Some tell of heated arguments in restaurants where diners insist that restaurants are required by law to provide free tap water. One says that under a law passed in Gladstone's time to promote temperance, restaurants are obliged to provide free tap water.

Some in the trade believe that they are required to serve free water. "As a retired hotelier and restaurateur, I was always led to believe that, by law, an innkeeper cannot refuse a traveller free water even if they do not spend any money at the establishment," says one poster on ccwater.org.uk.

But these assertions are pure hokum. The truth is that in Britain there is no legal requirement on restaurants to provide free tap water to customers. Some pubs and other premises permitted to sell alcohol may have a clause in their licence that does require them to offer free tap water, but it's on a localised basis only.

Sue Penniston, spokeswoman for the Drinking Water Inspectorate, part of Defra, says: "There are a lot of misconceptions, some of which arose out of the Leah Betts case. In the wake of her death, a lot of large premises had clauses put into their licensing agreements which obliged them to offer tap water free of charge.

"But apart from that restriction, restaurants do not have to give you water and do not have to supply it for free." She adds that it's nonsense for establishments to refuse tap water on health and safety grounds. "If a restaurant is on the public mains supply, the water is potable and there is no reason for it not to be supplied on health and safety grounds."

In the absence of legal compulsion, Buckland is encouraging good practice among restaurant chains. Pizza Hut has said it will always give customers free tap water. Strada, a pizza chain, already serves customers free chilled tap water in a designer bottle. "Strada is pleased to provide tap water free of charge throughout its restaurants as part of standard service," a spokeswoman says.

Restaurants and retailers are also facing intense pressure from green campaigners. Earlier this year, environment minister Phil Woolas condemned the bottled water industry as "morally unacceptable". Mineral water suppliers on average use two litres of water for every litre put into a bottle. Much is transported from overseas, from as far away as New Zealand and Fiji. Four out of five bottles are plastic, most of which end up in landfill despite recycling initiatives, where it can take four centuries to decompose.

Consumer campaigners Which? estimate that the number of plastic bottles sent to landfill each year would fill Wembley Stadium twice over. Which? describes bottled mineral water as an unnecessary drink that costs us £1.68bn a year. The good news is that sales fell by 9% last year, and in the credit crunch sales are expected to fall further. "Our reasons for buying bottled water are drying up," according to Which?

United Utilities, one of the biggest UK water companies, has begun a campaign called "tap into water" that brings free tap water to events such as football matches and street festivals. Philip Green, UU chief executive says: "At a time when we are facing such huge environmental challenges, buying bottled water at the rate of 3bn bottles a year in the UK is clearly unsustainable. I hope the hospitality industry will recognise their customers are environmentally aware and offer them free tap water without prejudice."

p.collinson@theguardian.com