The Prime Minister dismissed calls from a cross-party group of MPs to hold a wide-ranging Royal Commission to consider alternative methods, including legalisation.
After a year-long inquiry, the Commons Home Affairs Committee said the Government's current policy was not working.
However, speaking during a visit to Cambridge yesterday, Mr Cameron said: "I don't support decriminalisation. We have a policy which actually is working in Britain.
"Drugs use is coming down, the emphasis on treatment is absolutely right, and we need to continue with that to make sure we can really make a difference. Also, we need to do more to keep drugs out of our prisons.
"These are the Government's priorities and I think we should continue with that rather than have some very, very long-term Royal Commission."
In a report, the influential Home Affairs Committee said ministers could learn from the experience of Portugal where drugs have been "depenalised" – with possession of small amounts not subject to criminal penalties, even though they remain illegal.
It also urged the Government to fund detailed studies of changes in Washington and Colorado in the United States – where cannabis is being legalised – and Uruguay where a state monopoly of cannabis production and sale is being proposed.
Ten years after its predecessor committee last looked at the issue, it said change was now urgent and that a Royal Commission should be set up immediately so it could report back by 2015, when the next General Election is due to take place.