City officials said they were disappointed after nearly three quarters of premises tested sold alcohol to people under 18. Trading standards and police found 14 out of 20 places in Leicester sold alcohol to an underage mystery shopper.
Despite a warning, six of those bars, pubs or off-licences were then caught selling to the tester a second time.
The city council warned that three such offences in three months could lead to a �10,000 fine and licence suspension.
Officers said the businesses had been selected following information from the community.
All 14 businesses which broke the law have now been interviewed by police or trading standards officers and it is likely that legal action will follow.