The move follows an initiative which involved the company's senior directors serve behind the counter in the run-up to Christmas to experience the problems involved in refusing to sell alcohol to under-18s.
One of the participating directors, Chris Endersen, said: “We can see that it can be intimidating to face confident and mature-looking youths who attempt to purchase alcohol.
“It is, therefore, paramount that staff are trained to remain vigilant and to have robust procedures to detect and prevent potential underage sales.
“It is also critical for us to support them as they bear the brunt of this policing."
The company's 2,000 staff will be re-trained and company procedures will be re-drafted before March. All senior managers have already been through the Servewise training programme.
David Walker, chief executive of CJ Lang said: “Our efforts in relation to the prevention of underage drinking did not start with the arrival of test purchasing and its heavy penalties.
“We have, for years, been trying to protect our staff from mistakes which they can make relatively easily, particularly when under pressure from difficult customers."