The level of illegal drug use is not affected by how "tough" a country is when punishing those caught, a groundbreaking report has found, in what has been described as a "historic moment" in the battle to reform Britain's drug laws.
The Home Office's international study looked at different approaches to drugs policy and treatment in a number of countries, including some that have harsh criminal sanctions for users and some that have effectively decriminalised possession of drugs.
The study found no evidence that levels of drug use were affected by how "tough" or "soft" a government's response is, suggesting criminal sanctions have little impact.
The report also found positive health outcomes in Portugal, where possession of drugs is treated as a health matter rather a criminal issue, and no increase in use.
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Joao Goulao, Portugal's national co-ordinator on drugs, said decriminalising possession and use of drugs had cut addiction.
He told the Today programme: "The problematic drug of use, mainly the problems connected to heroin, we estimate that they have halved from the reality in the late 1990s.
"We had estimations of around 100,000 people hooked on heroin, which is 1% of our population, and now we estimate that we have half of that."
Danny Kushlick, founder of the Transform Drug Policy Foundation, said: "This is a historic moment in the development of UK drug policy.
"For the first time in over 40 years the Home Office has admitted that enforcing tough drug laws doesn't necessarily reduce levels of drug use.
"It has also acknowledged that decriminalising the possession of drugs doesn't increase levels of use.
"Even more, the department in charge of drugs prohibition says it will take account of the experiments in the legal regulation of cannabis in Washington, Colorado and Uruguay."
But the Home Office said the government have said they have "absolutely no intention of decriminalising drugs". The report is likely to stoke coalition tensions as the Lib Dem and Tory stances on the issue clash.
Clegg and Cameron have very different views on the issue of drugs law reform
David Cameron and Nick Clegg have previously clashed over Britain's war on drugs - with the Lib Dem leader challenging the prime minister to look at issues such as decriminalisation or legalisation of possession.
Norman Baker, the Lib Dem minister for crime prevention, is also an advocate of drug reform.
A Home Office spokesman said: "This government has absolutely no intention of decriminalising drugs. Our drugs strategy is working and there is a long-term downward trend in drug misuse in the UK.
"It is right that we look at drugs policies in other countries and today's report summarises a number of these international approaches."
Number 10 said there was "no chance" that "reckless" calls for decriminalisation would be entertained.
"This report provides no support whatsoever for the Lib Dem's policy of decriminalisation. In fact, it clearly states that it would be inappropriate to draw those kind of conclusions," they said.
"The Lib Dem policy would see drug dealers getting off scot-free and send an incredibly dangerous message to young people about the risks of taking drugs.
"As the report makes very clear, the Government's approach already provides a good balance between enforcement and treatment, drug use is plummeting as a result and there is simply no chance that we will entertain such a reckless change of course ."
The report is likely to add to calls from Liberal Democrats to scrap jail terms for possession of drugs for personal use.
The deputy prime minister earlier this year pledged to abolish prison sentences for the possession of drugs for personal use - including Class A substances like heroin and cocaine.
Mr Clegg has challenged David Cameron to look at issues such as decriminalisation or legalisation of drugs, despite the Prime Minister previously rejecting calls for a Royal Commission to consider the contentious issue.
A Home Office spokesman said: "This government has absolutely no intention of decriminalising drugs. Our drugs strategy is working and there is a long-term downward trend in drug misuse in the UK.
"It is right that we look at drugs policies in other countries and today's report summarises a number of these international approaches."
Mr Clegg accused the Conservatives of a "totally misplaced, outdated, backward-looking view" of the issue.
"The report this morning, pushed by me and the Liberal Democrats against resistance from the Conservatives, is the first time in a generation that a Government-commissioned report has shown the evidence that the way we are doing things doesn't make sense," he said.
A separate report calls for a blanket ban on supply of so-called legal highs.
Norman Baker said the government would look at the feasibility of one law to ban legal highs
Ministers are expected to unveil proposals for a blanket ban on all brain-altering drugs in a bid to tackle an epidemic of so-called legal highs.
The Government is to look at legislation introduced in Ireland four years ago that bans the sale of all ''psychoactive'' substances and then exempts some, such as alcohol and tobacco.
Currently, when a legal high is outlawed, illegal-drug chemists are getting around the law by tweaking the chemical compound and creating a new substance.
Home Office officials are to launch a feasibility study looking at the Irish model, which has effectively eliminated all so-called head shops that sell legal highs.
Crime prevention minister Norman Baker said: "From today we will start looking into the feasibility of a blanket ban on new psychoactive substances across the whole of the UK, clamping down on the suppliers and head shops rather than the users.
"This approach had a dramatic impact on the availability of 'legal' highs when introduced in Ireland, but we must ensure it would work here too."
Recently released figures showed there were 60 deaths related to legal highs in 2013 - up from 52 in 2012 and 29 in 2011.