THOUSANDS of consumers who order mineral water in restaurants are given filtered tap water and charged up to £4 a litre. It happens in top restaurants as well as in mid-ranking establishments. Diners who like still water are hit as hard as those who prefer it sparkling.

Restaurants serving tap water in place of mineral water include Antony Worrall Thompson’s London eatery and 192, the brasserie in Notting Hill. Chains such as Belgo, the Belgian mussel house, and Jamie’s bars are also cashing in.

Since first catching on in the 1980s, mineral water has become big business in Britain where the bottled water market is estimated to be more than £1 billion a year. With restaurant profit margins averaging about 500% per bottle, most waiters ask “Still or sparkling?” the moment a customer sits down.

“I resent paying through the nose for mineral water,” said Anna Pacey from Marylebone, London, last week. “That restaurants charge £3 or £4 for filtered tap water is outrageous.”

The restaurants install a filtration system to the mains water supply in their kitchens and the water is often served in flip-top reusable bottles stamped with their logo.

In all the restaurants visited by The Sunday Times last week, tap water was served after a specific request for “mineral water” and was charged at similar rates. Although none of the restaurants labelled the bottles as “mineral water”, which is a legally protected category, it is illegal not to tell customers that they are drinking tap water if mineral water has been requested.

There was no such explanation in the restaurants visited, although the bottles were labelled “pure still water” or “pure sparkling water” with “table water” in small print.

Mineral water companies are outraged by the practice, which could cost them millions of pounds in lost sales.

“It is highly misleading,” said Jo Jacobius, director of the Natural Mineral Water Information Service. “These bottles are often frosted and branded with a logo, which implies they’re something which they’re not.”

Jacobius said she had visited restaurants where waiters described filtered tap water as “in-house mineral water”.

Under European legislation mineral water must come from an underground source and be natural and free from additives. It must be tested to show it has no harmful microbiological impurities and that its mineral content is stable.

Mineral water is more expensive than filtered tap water mainly because costly equipment is required to extract it. While a bottle of mineral water typically costs a restaurateur 45p per litre, filtered water costs just 10p per litre.

The filtration systems, with reusable bottles, come from companies such as Classic Crystal, which has sold its £2,600 filtration system to restaurants, bars and hotels throughout Britain. It said last week that its bottles met legal standards and it had “no control” over how restaurants described the water.

However, a salesman told a reporter posing as a restaurateur that it was not necessary to tell customers they were getting filtered tap water. Asked if there was any need to tell people who ordered mineral water that they were being served tap water, he replied: “Oh no. Not at all.”

The company said that a filtration system could boost profits on water sales by up to 80%.

Unlike most of the restaurants surveyed, Worrall Thompson’s Notting Grill served its filtered water in carafes rather than bottles. It charged £1.50 for a litre, the least at any of the restaurants visited. George Muller, the manager, said the filtered water was better than some mineral water; it was served in carafes so customers were not confused and staff were supposed to tell diners what they were drinking.

Paul Gilchrist, operations manager of Belgo, which charges £1.75 for half a litre of filtered water, did not believe customers were being cheated. “We’re charging the going rate in the West End,” he said.

Tom Byng, owner of 192, said his staff told customers when they ordered mineral water that only filtered tap water was served. The restaurant did not have space to store bottled mineral water and it helped to cut waste. “We don’t do it to save money,” he said.

Jeremy Spencer, operations manager for the Hartford Group which owns 16 Jamie’s outlets in London, said the company would train staff to tell customers, or get tests done so the filtered water could be described as mineral water.

Filtered water, which is about the same quality as that dispensed by an office water cooler, is given away free by many hotels and restaurants such as Strada, the wood-fired pizza chain, which gives it free to diners.

Experts disagree about the health benefits of mineral water, which the industry claims is good for the digestive system. Recent reports suggest Britain’s tap water is as good for health as bottled water.

Additional reporting: Zoe Thomas, Will Iredale

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