Home Secretary Jacqui Smith has admitted she smoked cannabis while she was at Oxford University in the 1980s.
Her disclosure came the day after Prime Minister Gordon Brown said she would head a review of UK drugs strategy - including reviewing the cannabis laws.
"I did break the law... I was wrong... drugs are wrong," Ms Smith, 44, said.
She had smoked it "just a few times", had "not particularly" enjoyed it and had not taken any other illegal drugs, she told the BBC.
A number of senior politicians have admitted smoking cannabis in their youth - including at least one of her predecessors as home secretary, Charles Clarke.
Mr Brown's official spokesman said it was a "personal matter for ministers... there is no Number 10 edict on this one way or another".
He said the prime minister had repeatedly said he had never taken illegal drugs.
'Learnt my lesson'
Asked why today's students should listen when she urged them not to try the drug, she said that over the past 25 years the dangers of cannabis use had become clearer.
These included the mental health dangers of cannabis and the increasing strength of the drug.
Ms Smith said she did not think her admission made her unfit to be home secretary.
"I think in some ways I have learnt my lesson and I have a responsibility as home secretary now to make sure we put in place the laws and the support and information to make sure we carry on bringing cannabis use down, which we are doing," she told GMTV.
"On the whole I think people think human beings should do jobs like this. I am not proud about it, I did the wrong thing.
"One of the things about being a politician is that you are often criticised for not knowing what's going on. I hope that my experiences in my life have actually helped me understand that I do want crime tackled."
On Wednesday, the prime minister told MPs that laws introduced in 2004, making possession of cannabis a largely non-arrestable offence, could be reversed.
Ms Smith, who graduated from Hertford College in Oxford in 1984, went on to be a secondary school teacher before becoming one of the so-called "Blair babes" when she was elected Redditch MP in 1997.
Her appointment as home secretary was arguably the biggest surprise in Gordon Brown's first Cabinet, being promoted from her previous backroom role of labour's chief whip.
Past cannabis use has become less of an issue for politicians in the UK over recent years as a growing number have admitted using, or trying the drug while younger.
In 2000 then shadow home secretary Ann Widdecombe's call for £100 fines for cannabis possession led to eight members of the Conservative shadow cabinet admitting to smoking cannabis in the past.
Home Office minister Vernon Coaker last year admitted "when I was a student, I took one or two puffs of marijuana but that was it". Other past or present Labour ministers who have admitted cannabis use include Caroline Flint and Patricia Hewitt.
And on Thursday Home Office minister Tony McNulty said he had "encountered" and smoked cannabis once or twice when he was at university.
In 2004 the drug was downgraded from Class B, which includes things like amphetamines, to Class C alongside things like anabolic steroids, making it a largely non-arrestable offence.
The then home secretary David Blunkett said it was necessary to make drug laws credible and focus police efforts on the most dangerous drugs.
But the following year his successor, Charles Clarke, also considered reviewing the decision, after a link was suggested between cannabis use and mental illness.
Ms Smith is now to review whether it should be reversed in the light of research into links between cannabis use and mental health problems, and with crime.
A Home Office spokesman said the Advisory Council on the Misuse of Drugs (ACMD) will be asked to review reports that danger from cannabis is increasing due to wider availability of more potent strains such as "skunk".