Stores caught selling alcohol to under 18s on two or more occasions within a three-month period can now lose the right to sell alcohol for up to two weeks.
Amendments to the 2003 Licensing Act has seen the length of time that stores can be prohibited from selling alcohol extended from 48 to 336 hours.
Selling alcohol during a closure notice period is a criminal offence, punishable by up to six months in prison for the premises licence holder, a fine of up to £20,000, or both.
A closure notice can be served by trading standards officers or police, above superintendent rank, and can be enforced 14 days after it was served.
Retailers have a right to elect to go to trial should they wish to contend the notice.