The Global Commission on Drug Policy says it's "very weird" that Canada is taking a tougher line on marijuana when governments across the globe are reconsidering the war on drugs.

In an open letter Wednesday to Prime Minister Stephen Harper, the Brazil-based commission calls on Canada to stop pursuing the "destructive, expensive and ineffective" prohibition of pot.

Louise Arbour, a former Supreme Court of Canada judge, former Brazilian president Fernando Cardoso, former Swiss president Ruth Dreifuss and Virgin Group founder Richard Branson are among the signatories to the letter that warns Canada is repeating "the same grave mistakes as other countries."

"Building more prisons, tried for decades in the United States under its failed war on drugs, only deepens the drug problem and does not reduce cannabis supply or rates of use," says the letter. "Instead, North American youth now report easier access to cannabis than to alcohol or tobacco."

The commission includes an ideological cross-section of world leaders, among them George Shultz, former U.S. secretary of state in the Reagan Republican presidency, former Mexican president Ernesto Zedillo and Paul Volker, the former head of the U.S. Federal Reserve.

Their reservations are not theoretical, but are aimed directly at the Conservative government's omnibus crime bill currently nearing passage in the Senate.

The letter states that "with the proposed implementation of mandatory prison sentences for minor cannabis-related offences under Bill C-10, Canada is at the threshold of continuing to repeat the same grave mistakes as other countries, moving further down a path that has proven immensely destructive and ineffective at meeting its objectives."

Ilona Szabo, a spokeswoman for the commission secretariat, said in an interview that Canada is confounding an international community that considers the country a beacon of evidence-based policy.

"It's a very weird message for Canada because they would be ... discharging the evidence on the scene and they would be adopting policies that are outdated and that are actually much more harmful than the issues in question," Szabo told The Canadian Press in an interview from Rio de Janiero, Brazil.

She noted governments across the hemisphere are finally starting to reconsider their approach to drugs, just as Canada moves in the opposite direction.

Justice Minister Rob Nicholson, through his office, declined to be interviewed on arguments made by the commission.

"The Safe Streets and Communities Act goes after the source of the illicit drug trade — the drug traffickers," spokeswoman Julie DiMambro said in an emailed response. "The kinds of offenders we are targeting are those who are involved in exploiting the addictions of others for personal profit."

The email noted that the legislation includes "drug treatment courts so that those who are unfortunately addicted can get the help that they need."

But the commission argues that first-hand experience globally has shown that violent, illegal markets are the "inevitable" result of pot prohibition and can't be fixed with tougher laws.

"This has been the experience internationally," said the letter.

"In fact, among the things that are driving organized crime and violence in British Columbia and other Canadian provinces is, although on a lesser scale, just what is driving the violence in Mexico — demand for drugs in the United States. Tougher drug laws in Canada will not address this root cause."

And the letter-writers appeal to Canadians' sense of international pride to make their case.

"Canada has a proud international tradition of innovative and realistic policies; tougher drug law enforcement tactics such as mandatory minimum sentencing for minor drug law offences will put a huge strain on Canadian taxpayers, will not have the intended effect of creating safer communities, and will instead further entrench the marijuana industry in the hands of organized crime groups."

In a press release accompanying the release of the letter, Brazil's former president called for "fundamental reforms in national and global drug control policies."

"We should start by treating cannabis use as a health issue and undermine organized crime by legally regulating the drug's use rather than promoting prohibition policies which actually fuel gang violence, Cardoso said in the release.