Children were served booze in almost half of the pubs and clubs in Wyre where police carried out a series of spot checks.
Of the 21 premises in Fleetwood, Poulton, Thornton and Cleveleys which were targeted during the test purchasing operation, 10 failed.
Two children, aged 15 and 16, were even able to get past bouncers in some cases as they managed to buy alcohol at venues in each of the four towns.
In Poulton, five of the 10 places visited served the youngsters alcohol.
PC Lisa Evans, of Blackpool’s licensing team, said: “We had an awful lot of fails.
“They have all been spoken with and been made aware.”
The offending premises, which have not been named by police, have all received a £90 fixed penalty and will be revisited as part of a follow-up investigation in the next three months.
PC Evans said the results of the checks, which saw at least one premises fail in each of the four towns they visited, were “alarming”.
During the operation, the two youngsters went into pubs and clubs and tried to purchase alcohol.
Police officers, who accompanied them but did not enter the premises with them to avoid drawing attention to the youths, were then informed of the results.
PC Evans said: “Although it is somewhat alarming that so many venues have sold to underage customers, we hope this operation will highlight the need for staff to complete regular, comprehensive training related to underage sales.
“The protection of children from harm in licensed premises is paramount to the police licensing team and we hope it will be viewed as equally important in the licensed trade.”
The test purchasing checks were carried out by the licensing team on a Saturday evening last month.
Anyone who sells alcohol to someone under the age of 18 can be prosecuted and fined up to £5,000.