Deputy prime minister says politicians know 'war on drugs' is failing and prime minister must show courage over issue

Divisions between David Cameron and Nick Clegg over Britain's "war on drugs" emerged on Friday after the Liberal Democrat leader said that current policy was not working and accused politicians of "a conspiracy of silence".

Committing his party to pledging a major review of how to tackle the drug problem in its 2015 election manifesto, Clegg claimed Britain was losing the war "on an industrial scale". He said Cameron should have the courage to look at issues such as decriminalisation or legalisation of drugs.

Clegg threw his weight behind the report from the Commons home affairs select committee earlier this week that recommended that a royal commission look at such options.

"In politics, as in life, you can't keep on doing something that does not work," said Clegg, in an interview with the Sun. "You can't keep making the same mistakes.

"If you were waging any other war where you have 2,000 fatalities a year, your enemies are making billions in profits, constantly throwing new weapons at you and targeting young people – you'd have to say you are losing and it's time to do something different."

But Cameron told a press conference in Brussels: "Of course the deputy prime minister is entirely entitled to take a view for the next election and beyond for his manifesto, wanting to go further, wanting a royal commission.

"I personally don't support a royal commission. In my view there's always a danger, as someone said, that they can take minutes and last for years.

"I am very happy to debate and discuss drug policy. I think the coalition government has taken a series of good steps.

"The government, I think, has got a good record on these things and there is some good evidence that drug use and drug abuse is falling."

Ed Miliband was opposed to decriminalisation but said drugs policy needed re-examination. "We will look in detail at the home affairs committee report then come to a conclusion.

"What would be a problem would be to push into the far future any changes," said the Labour leader. "We need to look now at whether drugs policy is working. I am not in favour of decriminalisation but we will look at the report."

Clegg said he had already challenged Cameron on the issue. "I was disappointed that the Home Office ruled out an open-minded, level-headed look at all this before the ink had even dried on the committee report," he said. "I told the prime minister that this was a missed opportunity. He knows my views on this. He and I don't agree on this."

Clegg criticised attitudes at Westminster. "For too long, people in politics have worried that saying something differently can somehow look like you are being soft. It's important now to pluck up the courage to speak."

He said politicians knew the "war on drugs" wasn't working, but when in government they said everything was fine. "We've got to level with the British people and tell them what many people already know: it's time to do something different," he said.

Clegg has asked Jeremy Browne, a Lib Dem Home Office minister, to go on a fact-finding tour to look at approaches in Portugal, Amsterdam, the US and Latin America. He said the former Mexican president Felipe Calderón had privately admitted to him last year that his country's attempt to crush drug barons by military force had failed, claiming 60,000 lives. "He said to me: 'It's not working. We can't win against these odds.'"

Britain should be leading international debate, Clegg said, although he was not in favour of full legalisation. He said decriminalisation of possession while cracking down on traffickers and dealers might be a solution.

"Far from being soft, I want to get tougher on gangs who profit from the misery of drug addicts," he said.

Although the coalition government was doing "great things" on treatment, "we owe it to young people to find out what does work and then do it".

A Downing Street spokeswoman was asked whether taxpayers' money should be spent on sending Browne around the world to research changes to drug policy that Cameron opposed. "It is the government's role to keep policies under review and to ensure that they are meeting what is required, but the prime minister's view is that the policy is working," she said.

"It's part and parcel of a minister's job to make sure that they look at examples of policies in different countries. That's par for the course in many areas."

Danny Kushlick, of Transform, which campaigns for drugs policy reform, told the Sun Clegg should be congratulated for "telling us the truth about the war on drugs".