Millions of motorists could be driving while over the legal alcohol limit without even realising it.
A new Direct Line/MORI survey shows a third of motorists regularly drink and drive believing they are under the legal limit.
Yet, when questioned further, one in six of these - equivalent to 1.7 million drivers - admitted that they did not know how to calculate how many units they had consumed.
The survey also showed that nearly a third of the 2,000 people surveyed had driven in the past even though they were probably over the legal limit.
Direct Line road safety campaign manager Dominic Burch said: "Unfortunately there is no foolproof guide to how much alcohol you can drink and still stay within the law.
"It depends on the type of alcohol you have drunk, how much you weigh, your sex, age and metabolism.
"So the best advice if you are planning to drive is don't drink any alcohol at all."
A third of the drivers surveyed said they had been stopped and breathalysed at some point in their lives. One in ten admitted they had failed the test.
However, Direct Line said most drink-drivers learned their lesson once they had been caught.
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