Marijuana and cocaine for personal use should be decriminalised because the "war on drugs" has been a disaster, according to some of Latin America's most powerful politicians and writers.

The current international policy on drugs encourages corruption and violence that is threatening democracy throughout the continent, according to the former president of Brazil, Fernando Henrique Cardoso, who is a co-president of the Latin American commission on drugs and democracy. As well as politicians, the commission includes the writers Mario Vargas Llosa of Peru, and Paulo Coelho of Brazil.

The election of Barack Obama has opened up the best opportunity for decades to address the failure of the "so-called drugs war", Cardoso told the Guardian today on a visit to London. He said he was hopeful that the international community would acknowledge that the time had come for a "paradigm shift" in the debate on drugs. "The war on drugs has failed in spite of enormous efforts in places like Colombia – the area of coca crops is not reducing," he said.

The current system of prohibition encouraged corruption among police officers, politicians and even judges. "It poisons the whole system, it undermines democracy," Cardoso said. "The war on drugs is based on repression … How can people believe in democracy if the rule of law doesn't work?" Users should be offered treatment rather than jail, he said.

"The starting point has to be the United States," he said. "Now we have a new American administration, which is much more open-minded than before." He said he had held talks with the US state department in the later years of the Bush administration and found that, privately, many of the officials there shared his views.

Cardoso said that the changes would have to be co-ordinated. "We need an international convention, otherwise you will have different countries doing different things," he said. "But the climate is changing for the first time for many years. Even in the US, they recognise we are in deadlock now." Obama had already made it clear that the idea of a "war on drugs" was not workable. The need for change is urgent, said Cardoso, because of what is happening in Latin America. "There is a very grave situation in Mexico," he said. "More people are being killed there (through the drugs war) than in Iraq." He said that it was easier for former presidents who were no longer in office or running for election to speak out on such a controversial issue. He added that ending the war on drugs would be not be a signal that drugs were acceptable but a recognition that current policies had failed.

"You have to show that drugs are harmful, even light drugs, like marijuana - it is better not to use drugs - but tobacco is harmful also yet its use is being reduced by education," said Cardoso. He added that the vast quantities of money being used to enforce "repressive" policies on drugs could be put into treatment and education. Hundreds of thousands of people were being unnecessarily criminalised and sent to prison, "which are schools of crime."

The previous UN drugs policy that aimed to eliminate all drug use by this year was ill-conceived, he said. "You can never stop drugs use," he said, likening it to some of the failed policies in the past over HIV/Aids. "You can't have zero drugs any more than you can have a zero sex policy but you can have a safe sex policy." He said that Brazil's success in halting the HIV/Aids epidemic, which meant promoting the use of condoms in a Catholic country, was an example of how people's behaviour could be changed by education rather than repression.