Wine drinkers suffering an unexpected hangover after what they thought was a moderate drink may have just found someone else to blame but themselves.

A study of the alcohol content of 129,000 wines from vineyards across Europe and the new world over a 16-year period has suggested that many vintners have been “systematically” understating their wines’ strength on labels.

The American Association of Wine Economists found that 57% of the wines analysed were stronger than declared on the label. The average alcohol content was 13.6% when the average reported strength was 13.1% according to the biggest study of its kind undertaken yet. It was based on imports into Ontario, Canada, one of the few places to test the alcohol content of every incoming wine. Bottles from Chile, Argentina and the US were the worst offenders overall, but all of the wine-making countries analysed, including France, Italy and Spain, on average underplayed alcoholic power. Just under a third of the wines overstated their alcohol content and these were typically the weaker bottles.

The authors of the report, titled Splendide mendax: false label claims about high and rising alcohol content of wine, said informal discussions with winemakers showed they appeared to be reducing the alcohol levels printed on labels to improve sales in an era of rising strength level in wine caused by increased temperatures, changing tastes and improved wine-making techniques.

“Some winemakers … have admitted they deliberately chose to understate the alcohol content on a wine label, within the range of error permitted by the law, because they believed that it would be advantageous for marketing the wine to do so,” said the report, written by a team led by Julian Alston at the University of California.

“We observe systematic patterns in the errors: a tendency to overstate the alcohol content for wine that has relatively low actual alcohol content and a tendency to understate the alcohol content for wine that has relatively high alcohol content.”

Winemakers exporting to Britain are obliged to express alcohol in units of half a per cent but can claim a 14.5% bottle is 14% because the European regulations only require the label strength is not more than half a per cent out. Ontario allows a 1% tolerance.

“I think winemakers want to endow their wines with the sensory appeal associated with ripe flavours and concomitant high alcohol but know that many consumers are intellectually opposed to high alcohol levels,” said Jancis Robinson, a leading British wine critic. “They know that lots of people first ‘taste with their eyes’, by checking the alcohol level printed on the label.”

“Historically we haven’t worried about alcohol levels in wine too much,” said Jasper Morris, a master of wine at Berry Brothers and Rudd, the Queen’s wine merchant. “It used to be all around 12% to 13%. It certainly wasn’t like the difference between buying lager, where premium brands can be twice as strong.But labelling is beginning to become more of an issue when you have wines at 14.5% and 15%. Like everybody else, I want the product to be the same as what it says on the label.”

Michael Cox, UK director of Wines of Chile, denied there was a systematic approach to under-reporting alcohol content in Chilean wines and said there were some cases where consumers were attracted by strength. But he added: “There is a trend towards lower alcohol wines and so to some extent there is a marketing angle.”

The analysis also revealed that strength of wine across the world has risen by almost a whole percentage point in recent years. Between 1992 and 2006, the average alcohol in a bottle of wine that passed through Ontario rose from 12.6% to 13.6%. In some cases alcohol levels have risen to double that. American wine is typically almost one percentage point stronger than their European counterparts and Australian wine half a percentage point. But some of France’s prestigious wines are now producing their own blockbusters. Last week, Chateau La Mission Haut-Brion in Bordeaux released a £580 bottle of wine with 15% alcohol.

Tests on individual wines present a mixed picture. Jon Bonné, the wine critic of the San Francisco Chronicle, ordered tests on 19 wines and found that 14 had understated alcohol levels, three by a whole percentage point. The Guardian this week commissioned analysis on six bottles on sale in Majestic from Australia, California, France and Italy. The tests were conducted using the EU standard distillation method and the analyst was able to check the bottles to a tolerance of 0.3 percentage points. Taking that into account, none breached EU regulations but in each case customers were getting less alcohol than they bargained for, not more.

The true extent of inaccurate reporting remains unclear in the UK. The Food Standards Agency which is charged with ensuring labels on imported wines do not make false claims only carries out spot checks on non-EU wine and then only checks against the alcohol level provided on official documentation provided by the winemaker and does not carry out its own alcohol analysis.

Pricey vintage

The 2010 Bordeaux vintage is the most expensive ever, according to wine merchants. Many of the Grands Crus Classés, including Chateau Palmer and Chateau Pontet Canet, have announced record prices, higher even than for their 2009 wines, which was declared by critics the “vintage of the century”. Ausone from St Emilion went on sale last week at £1,030 a bottle. Mouton Rothschild is £650 a bottle, but within 25 minutes of going on sale wine merchant Berry Brothers and Rudd (BBR) sold 500 cases. Even Lynch Bages, a fifth growth which suggests it should be inferior, is on sale at £1,250 a dozen, up from £865 for the 2009.

“It is without doubt the most expensive vintage ever,” said Gary Boom, director of Bordeaux Index.

“The chateaux have priced it very high and I believe they have misjudged the mood.”

The quality has inspired euphoric responses. “It is exceptionally unctuous, silky, almost with the consistency of single cream,” said Simon Staples, sales director of BBR, of Mouton. “Incredible, ripe, but not overly so, with red and black fruits, and a really dense compote of woodland lushness.

“The tannins are gloriously wrapped up within the satin creaminess of its dense but elegant body and it has an extraordinary finish of minutes not seconds.” www.Wine Business International.com