Actors pretending to be drunk were served alcohol illegally in 84% of cases, researchers at Liverpool John Moores University found.
UK law preventing the sale of alcohol to drunks is routinely being broken, placing "enormous burdens" on health services, the study concluded.
Four actors were trained to act drunk using slurred speech, unsteadiness, lack of eye focus and fumbling.
They were served more alcohol in most of 73 bars tested in a north-west city.
Often bartenders recognised they were drunk - either by comments or by rolling their eyes - but still suggested they buy double rather than single shots in 18% of cases.
Researchers at the university's World Health Organisation Collaborating Centre for Violence Prevention said 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 report, published in the Journal of Epidemiology and Community Health, stated: "UK law preventing sales of alcohol to drunks is routinely broken in nightlife environments, yet prosecutions are rare.
"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. Preventing alcohol sales to drunks should be a public health priority, while policy failures on issues, such as alcohol pricing, are revisited."
The attempts to buy alcohol were made from Wednesday to Sunday between 21:00 and 03:00.
The study found that service rates were "always high", ranging from 60% on Wednesdays to 94% on Fridays, and from 78% before midnight to 96% after midnight.
Some 95% of attempts to buy alcohol were successful in venues where there were bouncers, compared with two-thirds in those without bouncers.
Norman Baker, crime prevention minister, said knowingly selling alcohol to someone who was drunk was illegal, and "anyone who witnesses this irresponsible behaviour should report it".
It is an offence under the Licensing Act 2003 to knowingly serve alcohol to a drunk person, or to obtain alcohol for a drunk person on a licensed premises.