The amount of alcohol people in England drink has been underestimated by the equivalent of 12 million bottles of wine a week, according to new research.
In England, surveys measuring typical drinking habits account for only around 60% of alcohol sold, the medical journal BMC Medicine report said.
Report author Dr Mark Bellis said this was because many studies do not include drinking on special occasions.
More than 6,000 people in England were interviewed for the study.
Accounting for special occasion drinking added more than 120 million UK units of alcohol - equivalent to about 12 million bottles of wine - to the population's alcohol consumption in England every week, it found.
The results could have important implications for public health, researchers said.
"Nationally, we underestimate how much we drink - and as individuals we can turn a blind eye to our heavier drinking periods when we calculate personal consumption," said lead scientist Dr Bellis, from Liverpool John Moores University.
"For many people, though, these sessions add substantial amounts of alcohol to their annual consumption and inevitably increase their risks of developing alcohol-related ill health."
The equivalent of more than three-quarters of a bottle of wine (or about three pints of beer) per drinker every week goes unaccounted for, he said.
The survey measured a medium glass of 12.5% ABV wine as 2.2 UK units, and a 440ml can of 4.5% ABV beer as 2 UK units, but the amount of alcohol units in drinks varies depending on their size and strength. The NHS has a guide to calculating alcohol units.
Researchers conducted telephone interviews with 6,085 randomly-selected members of the public aged 16 and over in England.
Participants were asked about normal drinking patterns and those outside their usual circumstances, such as summer holidays, bank holidays, and weddings.
Most categories of drinkers, based on age groups and levels of typical consumption, reported increased consumption during holidays or special occasions.
The biggest increase was seen in 25 to 35-year-olds, who had the highest level of typical consumption.
People in this drinking category drank an extra 18 units (144g) of alcohol per week on special occasions, the research suggested.
Last year, the Organisation for Economic Co-operation and Development found that people over the age of 15 in the UK drank an average of 10.6 litres of pure alcohol a year - equal to 115 bottles of wine.
According to the Institute of Alcohol Studies, in 2012, men consumed an average of 17 units in the week before they were interviewed, compared with 10.2 units for women.
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James Nicholls, from the charity Alcohol Research UK, said: "Patterns of consumption have a significant influence on the health impacts of alcohol.
"For instance, it is widely recognised that any protective effects of moderate drinking on the heart are cancelled out by heavy drinking episodes.
"If we can better quantify where peaks in consumption occur, among which groups of drinkers, and at what scale, we will be much better placed to target interventions aimed at reducing harm."