The majority of restaurants "push" costly bottled water on diners rather than providing free tap water, a consumer group has said.
Tap water is more environmentally-friendly, campaigners say
A fifth of customers feel inhibited asking for tap water, a National Consumer Council survey suggested.
Carl Belgrove of the NCC said that most British restaurants "push mineral water just to make a quick profit".
But the Bottled Water Information Office said the vast majority "are only too happy to offer that choice".
The NCC said it was calling for an end to what it calls the mineral water rip-off.
According to its survey, 70% of those questioned thought mineral water in restaurants was too expensive and wanted tap water to be available for free.
Just 1% of restaurants in north-west England and London always offer free tap water over mineral, the study found.
Mr Belgrove said that free tap water was environmentally friendly because it reduced the number of discarded plastic bottles.
Market researchers TNS spoke to 1,000 people over the age of 16 in September for the survey.