Half of consumers wouldn't ask for a service charge to be reduced or removed from their bill if they had poor service in a restaurant, research has found.
The survey by The Good Food Guide 2009, which questioned 1,335 people, found that three quarters of diners tip sometimes, with 85% of them varying the amount depending on the quality of service.
One in four people tip on top of added service charges, the survey found, but such double tipping is often the result of a credit card slip or machine being left open for a tip despite a service charge having already been added.
More than half of people surveyed said they usually leave tips of 10% or more, with one in five choosing smaller amounts, of around 5%, even if they receive good service.
The south of England has the most generous tippers with 62% of diners leaving 10% or more, while the worst tippers live in Northern Ireland, where just 31% of people tip this amount.
The Good Food Guide editor Liz Carter said: "Most restaurants say service charges are optional, so customers have every right to deduct them if they haven't had good service. But many are reluctant to put themselves in such awkward situations."
Earlier in the summer, The Government confirmed that it will outlaw the practice of topping up minimum wage with tips.
Despite warnings from industry that many staff will actually be worse off because of added national insurance payments, the Department for Business, Enterprise and Regulatory Reform announced that all staff must be paid at least the £5.52 minimum wage as basic pay.
By Kerstin Kühn
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