Doncaster has became the first town to officially adopt the trade's new Challenge 21 scheme to cut under-age sales in pubs.
Doncaster's step comes ahead of a national roll-out of the scheme, which aims to deliver new proof-of-age signage into every on-trade premises in the country.
The launch follows a similar scheme adopted in the off-trade last year.
The thrust of the initiative is to impress on youngsters that if they are "lucky enough" to look under 21 they will be asked to prove they are over 18.
The scheme is spearheaded by the British Beer & Pub Association (BBPA), which has produced special posters and signage for licensees and operators.
Northern BBPA secretary Lee Le Clercq said: "Challenge 21 is designed to place far more emphasis on ID checks and make young people get used to the fact they are going to be asked for ID."
The new posters stress that a photocard driving licence, passport or Proof Of Age Standard Scheme card with an official hologram are the only acceptable forms of identification.