Stores caught selling alcohol to children twice within three months will now lose their licences. Previously a “three strikes” rule has applied.
Local councillors have been given new rights to call for licence reviews of premises they feel are problematic.
But there are also measures aimed at children themselves. Police in England and Wales can now confiscate drinks from under-18s without having to prove they intended to consume it.
And police can also disperse groups of children as young as 10 from public places. The previous regime had only applied to over-16s.
A new offence for children repeatedly being caught with alcohol has also come into force.
Home Office Minister Alan Campbell said: "The majority of young people are model citizens but there are a minority that are not. These powers will make it easier for police to take tough action against those groups whose behaviour can affect a whole community.
"Alongside this we are challenging young people's attitudes about binge drinking. Our Know Your Limits campaign continues to make people think about how much they are drinking."
The new measures, introduced in the Policing & Crime Act, are part of a government strategy to tackle under-age drinking set out in the 2008 Youth Alcohol Action Plan.