The force sent 15-year-old volunteers, who they say looked their age, into ten pubs and bars and were served in three of them.
Sergeant Dave Shaw branded the 30% failure rate as ‘completely unacceptable.’
“It shows that licensed premises need to be doing more to comply with the law and prevent children from buying alcohol. Children and alcohol is a dangerous mix that affects the health of our young people and makes them more vulnerable to crime. It also increases anti-social behaviour in our neighbourhoods, and pubs and bars must do more to prevent this.
"We are going to be placing an increased emphasis on test purchase operations on public houses and bars throughout Leeds this year and during 2016 after previously focusing on off-licences.
"Any premises who are found selling to children twice within a three month period risk having their licences removed by the council."
Staff members who served the underage volunteers have been given community resolutions, a way of dealing with minor crime that can include writing an apology, completing some unpaid work or rehabilitative activity.
Five Brighton venues were caught selling alcohol to underage volunteers earlier this years, which was described by local police as ‘very disappointing.’