Sunderland South police have asked
retailers to sign up to the voluntary agreement, which would apply on Fridays and Saturdays.
Humphreys said the concept went against the law and would lead to confusion among customers and retailers.
He said: "I don't feel much enthusiasm for the idea. The law should be followed and if the law says it's legal for 18, 19 and 20-year-olds to buy alcohol then they should be allowed to.
"How on earth do you train the people who work in retail if you expect them to move from one age to another? It should be about educating staff to ask for ID and not about refusing to serve the responsible 18, 19 and 20-year-olds."
by Sunderland South's neighbourhood policing team to cut anti-social behaviour include confiscating alcohol and carrying out test purchases.
Sergeant Steve Lavelle said: "It's well known that youth disorder is often linked to alcohol and under-age drinking, and while disorder has reduced in the area it continues to blight peoples' lives.
"We are determined to stop this and urging off-licences not to serve alcohol to anyone under 21 each Friday and ≠Saturday is just one of the steps we're taking."