Pubs can now apply for licences to open for up to 24 hours ahead of changes due to come in later this year.
Opponents fear the changes will encourage binge drinking
The government says more flexible hours will stop drinkers spilling onto the street all at once and says few pubs will apply to remain open for 24 hours.
But the plans are opposed by some police chiefs, medics and Tories, who are worried about binge drinking.
The changes for England and Wales come into force in November, but the Tories and Lib Dems want them put on hold.
Over the next six months venues will be able to apply for extended licences but will have to inform the local community to give them and the police 21 days to object.
A snapshot survey by the BBC suggested most would apply for an extra one or two hours, usually at the weekend.
The government says current licensing laws are outdated and the Licensing Act 2003 will mean a bigger say for residents and new powers for police to deal with drunken disorder.
It has outlined plans to give disorderly pubs eight weeks to clean up their act before being billed for extra policing.
"The vast majority of people should be treated like the adults they are," said Culture Secretary Tessa Jowell.
"It is ridiculous that the government should deny the entire population the right to a drink after 11pm. We will give adults the freedom they deserve and yobs the tough treatment they deserve."
But Lib Dem spokesman Don Foster said "cracks" were appearing in government efforts to tackle binge drinking.
"The tragedy is that properly managed, more flexible licensing hours could help reduce the problems caused by binge drinking and alcohol-related violence."
Mr Foster made a renewed call for a delay on the implementation of the new licensing regulations.
Former Metropolitan Police Commissioner Sir John Stevens said ministers' plans would mean more officers would be moved from other duties to cover the streets in the early hours.
The Royal College of Physicians said the plan "flies in the face of commonsense".
And Dr Robert Lefever, director of PROMIS addiction centre, said: "This will totally fail - binge drinkers will simply binge for longer."
In January members of the British Beer and Pub Association were asked whether they wanted to remain open longer when flexible opening hours come into force.
Of the 20 respondents, none said they would apply for 24-hour licences.
The reluctance to stay open round-the-clock was largely due to the costs involved.