Although the group's card was the first national proof of age card to be introduced, the rise in popularity of others, such as the CitizenCard and Validate UK, has meant fewer people are applying for it.
Portman proof of age card application packs have not been sent out to retailers since May and, after Sept 30, applications received by them will be returned to the applicant.
Portman Group chief executive David Poley said: "Over the years, it has helped half a million young people to prove their age. But with the drop in demand, the time is right to stop issuing new cards. We can now concentrate on raising standards of drinks producer activity."
Poley stressed that cards already in existence should still be accepted as ID by retailers.
There are currently 18 proof of age cards, including the Portman card, which have been endorsed by PASS, the government's national proof of age standards scheme.
Alongside national cards there are many issued by councils including Blackpool, Herefordshire and Southampton, featuring the official PASS hologram and therefore recognised as valid forms of ID.
Robert Humphreys, chairman of the PASS board, said the Portman Group had been a "model of good practice" and had helped both the trade and young people to work within legal boundaries.
He added: "Any card bearing the PASS hologram is reliable and should be accepted as genuine, and the standards applied within the PASS scheme were largely developed by the Portman Group as pathfinder."