More teenagers will die from liver disease if 24-hour drinking goes ahead, two of the UK's top doctors have said.
In 2003-2004, 4,647 under-18s were admitted to hospital.
Longer opening hours will cause a rise in cirrhosis among young people, Ian Gilmore and Kieran Moriarty warned.
Government plans to liberalise drinking laws from November were "insane", the doctors told the Times newspaper.
But the Department for Culture, Media and Sport said the new laws would give police extra powers to deal with businesses serving underage drinkers.
A spokesman said: "We share doctors' concerns about the rise in drinking-related disease among young people.
"That's why the Licensing Act increases police powers to deal with businesses that serve alcohol to underage drinkers."
But he said excessive alcohol consumption often took place in the home and there was no evidence previous changes in UK licence led to increased drinking.
Culture Secretary Tessa Jowell has promised to keep the relaxation of licensing laws under review.
Professor Gilmore is the chairman of the Royal College of Physicians' alcohol committee and Dr Moriarty advises the Department of Health on liver disease.
They urged the government to reduce young people's access to alcohol.
Professor Gilmore told the Times: "Worldwide research shows that levels of consumption are heavily increased by price and availability.
"An increase in hours of sale is likely to be associated with a rise rather than a fall in alcohol consumption."
According to Royal College of Physicians figures, one in three 15-year-olds admit to having been drunk at least once.
Dr Moriarty told the BBC: "Now we're seeing patients in their thirties and twenties and even the occasional patient in their teens will irreversible alcoholic liver disease and dying in their twenties with irreversible alcoholic liver disease - with jaundice and liver coma, internal bleeding - and it's very distressing."
Government figures released at the start of the month showed the number of UK children admitted to hospital because of drinking alcohol had risen by 11% since the mid-1990s.
In 2003-2004, 4,647 under-18s were admitted to hospital, the equivalent of 13 a day - up from 4,173 in 1996-1997.