The government has been defeated in the Lords after peers backed a Conservative motion to delay the start of extended pub licensing hours.
The vote to delay the reforms was seen as a warning to ministers
The Licensing Act, which allows round-the-clock drinking, is due to come into effect on 24 November.
On Monday, peers voted by a majority of 33 for a motion calling on the government to think again.
The defeat does not mean the new system will be delayed or scrapped, but is the biggest rebuke the Lords can use.
Licensed premises can apply to open beyond 11pm from next week.
Pressure for rethink?
Tory and Lib Dem peers said the act's implementation should be delayed until June 2006.
The vote will be seen as increasing the pressure for a rethink.
The motion urged the government to delay implementation of the extended opening hours "to allow more time to address public concerns about the effects of the proposed changes".
Viscount Astor, for the Conservatives, told peers the policy was "a farce, a total mess".
The government had "mucked up the introduction" of extended licensing hours.
Lord McNally, the Lib Dem leader in the Lords, said opponents of the government's plans were not "killjoys", but the changes could cause a "nuisance".
"This is clearly an issue which requires further thought and study," he added.
But Lord Davies, for the government, accused opponents of making "reckless arguments" and said a delay would "produce chaos for the industry and the public".
Putting back the date would leave premises unlicensed, he added.
Government officials say attempts to delay the changes would cause problems in the run-up to Christmas and threaten thousands of jobs.
Earlier, Conservative family spokeswoman Theresa May cited a warning by the British Beer and Pub Association that thousands of pubs may be forced to open "illegally" on 24 November because their new premises licence has not arrived.
"We are not going to delay the Licensing Act," said a spokeswoman for the Department of Culture, Media and Sport.
"The arguments are well-rehearsed: a delay would deny police tougher powers to deal with drunken disorder.
"It would deny pubs a more effective voice in licensing decisions that affect them and it would mean the continuation of an unjust 90-year-old curfew that punished the responsible majority for the sins of the irresponsible minority.
"That is not to mention all the work done and fees paid by hundreds of thousands of village halls, sports clubs, members' clubs, pubs, restaurants and bars that would be wasted."
A nationwide poster campaign warning of on-the-spot fines for disorder is set to be launched by ministers on Tuesday.
The posters will say "get drunk and disorderly, get arrested, get an £80 fine".