The Independent on Sunday has carried a front-page apology for its 10-year campaign to legalise cannabis.
Police in London have found 1,500 cannabis farms in just two years
The newspaper says it has changed its stance in the face of growing fears over addiction to the drug.
There are now more than 22,000 people a year, almost half under the age of 18, being treated for cannabis addiction.
And the paper says mental health problems and psychosis affect thousands of teenagers who use high-strength cannabis, known as "skunk".
When the Independent on Sunday's then editor Rosie Boycott launched a campaign to decriminalise cannabis in 1997, 16,000 people marched through central London to support it.
Then, the paper says, it was "leading a consensus", but now its editorial says that "the growing evidence of the risk of psychological harm" has forced it to do a U-turn.
The Police Federation's Jan Berry told BBC One's Sunday AM programme she welcomed the apology.
"Many people only read one newspaper or maybe a couple of newspapers and their views of the world are formed by what you put in those papers," she said.
"So 10 years ago, the Independent on Sunday said very big that cannabis should be legalised, should be decriminalised. And today they're putting in an apology and I think that's great if they acknowledge they've done wrong."
The paper says that in 1997 there were just 1,600 people in the UK being treated for cannabis addiction compared to many times that now.
It quotes several senior scientists, including the head of the Medical Research Council, Professor Colin Blakemore, who backed the campaign, but has now changed his mind.
Professor Robin Murray, from the London Institute of Psychiatry, also tells the paper that at least 25,000 of the UK's 250,000 schizophrenics could have been spared the illness if they had not used cannabis.
And the paper points to research to be published in this week's Lancet which says cannabis is more dangerous than LSD and ecstasy.
In 2004, the government downgraded cannabis from a class B to a class C drug.
The Independent on Sunday says it believes the current classification and level of police enforcement is "about right".
"The fact that the possession of cannabis - and other drugs - is illegal acts as an important social restraint," it adds.
The paper says skunk smoked today contains 25 times more of the active ingredient than was typically found in cannabis during the 1980s.
It also says cannabis is more easily available, having fallen in price from about £120 an ounce in 1994 to £43 today.