Middle-aged men who drink the equivalent of two and a half pints a day risk speeding up memory loss and mental decline by six years, researchers have warned.
A 10-year study of more than 7,000 male and female civil servants found men who regularly drank significant amounts of alcohol suffered notable mental decline.
In contrast, men who drank less than one and a half pints a day, or a glass and a half of wine, suffered no impact on mental ability.
The heaviest drinkers were found to have the equivalent memory of someone in their early 70s even though they were still in their mid-60s, researchers found.
The study proved that some of the memory loss experienced in old age may have been caused by unhealthy drinking habits in middle age.
Dr Severine Sabia, from University College London, said: "Much of the research evidence about drinking and a relationship to memory and executive function is based on older populations. Our study focused on middle-aged participants and suggests that heavy drinking is associated with faster decline in all areas of cognitive function in men."
The volunteers were tested between 1997 and 1999 when they had an average age of 56, and retested 10 years later.
To test their memory they were given two minutes to recall a set of 20 two-syllable words. They were also asked to complete a series of reasoning and problem-solving tests. All male heavy drinkers showed a decline in memory, reasoning and problem-solving of at least 1.5 years but in the most severe cases, mental ability had declined by nearly six years.
Dr Sabia added: "This study suggested that men consuming high levels of alcohol in midlife were more likely to experience a faster 10-year cognitive decline in all cognitive domains.
"Our findings are in agreement with previous studies that moderate alcohol consumption is probably not deleterious for cognitive outcomes, but they also show that heavy alcohol consumption in midlife is likely to be harmful for cognitive ageing, at least in men."
However the study also found that women who completely abstained from alcohol also suffered mental decline. "In women there was only weak evidence that heavy drinking was associated with a faster decline in executive function, but abstinence from alcohol was associated with faster decline in the global cognitive score," said Sabia.
Guidelines from Britain's NHS currently advise there is no guaranteed safe level of drinking but recommend no more than three to four units a day for men - the equivalent of two normal strength pints of beer. Women should drink no more than two to three units.
Figures show that men who drink more than that limit are twice as likely to get cancer of the mouth, neck and throat, while women are 1.2 times as likely to get breast cancer.
The study was reported in the journal Neurology.