Pubs and nightclubs are routinely breaking a law which prevents them serving alcohol to drunk people, research shows.
Experts found that 84% of attempts by actors to buy drinks while pretending to be intoxicated resulted in a barman or woman serving them.
This is despite the fact the bartender often recognised drunk behaviour - either by commenting directly to the actor or another bartender, or by rolling their eyes.
Researchers said the issue should now become a public health priority, arguing that preventing sales of alcohol to already drunk people would protect long-term health and reduce the strain on the NHS and other public services.
The study, published in the Journal of Epidemiology and Community Health, was carried out by a team at the World Health Organisation Collaborating Centre for Violence Prevention at Liverpool John Moores University.
Four student actors (two male, two female, aged 20 to 22) were recruited through auditions and trained on acting drunk.
Their abilities were tested on police, who regularly deal with drunk people, and their behaviour included slurred speech, being unsteady on their feet and having difficulty focusing.
All attempts to buy alcohol were made from Wednesday to Sunday evenings between 9pm and 3am.
The actor pretending to be drunk stumbled to the bar and used "loud, slurred speech" to ask the price of a vodka and coke.
He or she then fumbled over their purse or wallet and asked for the drink.
The study found that service rates were "always high", ranging from 60% on Wednesdays to 94% on Fridays, and from 78% served before midnight to 96% after midnight.
Overall, 73 randomly selected pubs, bars and nightclubs were tested in one city in the North West of England for the study.
In 18% of sales, barmen and women attempted to up-sell by suggesting actors purchased double rather than single vodkas.
The authors said: "UK law preventing sales of alcohol to drunks is routinely broken in nightlife environments, yet prosecutions are rare (three in 2010).
"Nightlife drunkenness places enormous burdens on health and health services.
"Preventing alcohol sales to drunks should be a public health priority, while policy failures on issues, such as alcohol pricing, are revisited."
The study also found that those venues that had indicators of poor management - such as cheap alcohol promotions, young bar workers, young clientele, crowds, poor lighting, rowdiness, dirtiness and high levels of drunkenness - were most likely to serve the fake drunks.
Some 95% of attempts to buy alcohol were successful in venues where there were bouncers, compared to two-thirds in those without bouncers.
The authors said: "Although our study focused on one city, a lack of prosecution for sales to drunks throughout England suggests this is typical of nightlife environments nationally.
"With policies to prevent alcohol-related harm by increasing alcohol prices failing to be implemented, increased use of legislation preventing sales of alcohol to drunks should be considered a public health priority."
Eric Appleby, chief executive of Alcohol Concern, said: "While it's disappointing that some bars are serving drunk people it's not hugely surprising as bar staff are unlikely to have received adequate training to help them identify and deal with a drunk person in a safe and appropriate way.
"We need to focus on developing a culture where it's not seen as normal or acceptable to be drunk. To do this we have to stop selling cheap booze, tackle the fact that it's available everywhere and get tough on alcohol advertising."
Crime Prevention Minister Norman Baker said: "Knowingly selling alcohol to someone who is drunk is illegal under the 2003 Licensing Act and anyone who witnesses this irresponsible behaviour should report it.
"The alcohol industry and those who sell alcohol have a duty to sell it responsibly."