The security detail at the presidential offices in Bogotá was understandably heavy. Armed police and the military were much in evidence as President Juan Manuel Santos of Colombia hosted the leaders of Bolivia, Ecuador and Peru for a regional economic summit. The security forces outside the Palacio de Nariño in the city centre had extra reason to be on high alert – the summit last Tuesday came only days after Colombian special forces shot and killed Alfonso Cano, leader of the Farc (Revolutionary Armed Forces of Colombia) guerrilla group.

It was a major success for Santos, an increasingly influential figure in Latin American politics. The former Brazilian president, Lula da Silva, said recently that Santos, who comes to Britain in two weeks, was assuming the mantle of a continental leader.

Da Silva's compliment is true in at least one respect, since Santos has emerged as the leading voice on the international political stage calling for a major rethink on the war on drugs. Santos's call for a new debate about drug regulation is heavily symbolic, since Colombia has suffered more than any other country at the hands of narcotics traffickers.

Santos has drawn attention to the damage suffered by the producing nations in Latin America as they continue to serve the growing demand for drugs in the consuming nations of the west. His voice is becoming the key one in trying to set the terms for a new international discussion about the war.

Santos, an urbane, affable 60-year-old, who was elected last year, is well placed to lead the global debate. He is a keen internationalist and was educated at Harvard and the London School of Economics. One colleague described London as his "dream city". His visit to Britain will be part of his attempt to rebrand his country – from the failed state of 10 years ago to an emerging economic powerhouse in Latin America. The killing of Cano is the latest stage in that journey.

Santos's response in the days since Cano's death – widely described in the local media as the most significant blow to the Farc – has avoided triumphalism. And with good reason. In the month leading up to Cano's assassination, more than 20 soldiers were killed by the Farc. With those deaths came the first signs of public disquiet that the security gains made in the previous 10 years were starting to slip away. As the leading political magazine, Semana, said: "The killing of Cano couldn't have come at a better time for the government."

The Farc emerged in the mid-60s as a Marxist-Leninist group determined to overthrow a state which it saw as riven by inequalities, one where power and high office, both economically and politically, was dominated by an elite group of families. Farc's leftwing ideology was driven by a clutch of university-educated young men and women and was part of a wider movement in Latin America where revolutionary groups were taking up arms in search of social justice and in response to the grinding poverty and gross inequalities across the continent.

At the height of its power in the 1990s the Farc controlled a third of Colombian territory. Now that it is much reduced and scattered to the remotest parts of the country in the wake of the decade-long military offensive, many Colombians believe the Farc's ideological fervour has mostly disappeared. But in its place has come an increasing appetite for drug trafficking.

Colombia is now emerging from its darkest days of guerrilla and narcotics warfare. It is attracting ever more foreign investment to its born-again cities of Bogotá and Medellín. Where strife and terrorism were once routine, there are now real signs of a civic and economic revival – cities being regenerated, booming tourism and impressive growth rates. It is this economic story which Santos will relay on his visit to Britain.

But Colombia's recent history still bears the deep scars of its battle with drugs. As Santos says: "We dismantled the drug cartels. Those big cartels that had our democracy on its knees – they no longer exist. The only big cartel still is the Farc but we have weakened them more and more."

It is in this context – as the president of a country that was very nearly broken by a combination of drug cartels and guerrilla narcotics traffickers – that Santos's recent pronouncements on the war on drugs are all the more remarkable. Last month he said: "The world needs to discuss new approaches… we are basically still thinking within the same framework as we have done for the last 40 years."

Santos has gone further than any other leading politician in opening up the debate. In an interview with the Observer he spelled out the radical ideas which he hopes will create a fresh approach. He said: "A new approach should try and take away the violent profit that comes with drug trafficking… If that means legalising, and the world thinks that's the solution, I will welcome it. I'm not against it."

But he is clear that any initiatives need to be part of a co-ordinated international plan of action and he rules out any unilateral action by Colombia. "What I won't do is to become the vanguard of that movement because then I will be crucified. But I would gladly participate in those discussions because we are the country that's still suffering most and have suffered most historically with the high consumption of the UK, the US, and Europe in general."

Santos is prepared to go much further than others – he is opening up a debate about legalising marijuana and perhaps cocaine.

"I would talk about legalising marijuana and more than just marijuana. If the world thinks that this is the correct approach, because for example in our case we used to be exporters, but we were replaced by the producers of California. And there even was a referendum in California to legalise it and they lost it but they could have won. I ask myself how would you explain marijuana being legalised in California and cocaine consumption being penalised in Idaho? It's a contradiction. So it's a difficult problem where you set the limits. It's a difficult decision. For example, I would never legalise very hard drugs like morphine or heroin because in fact they are suicidal drugs. I might consider legalising cocaine if there is a world consensus because this drug has affected us most here in Colombia. I don't know what is more harmful, cocaine or marijuana. That's a health discussion. But again, only if there is a consensus."

Santos is not alone. There is a growing impatience in the producing countries of Latin America that suffer acutely as their drug cartels feed the demand in the consuming countries.

For Santos and his country, the issue of drugs looms much larger than for the consuming nations. For Colombia, drugs are "a matter of national security" whereas, for others, "it is mainly a health and crime issue". He speaks eloquently of the price his country has paid – and continues to pay – for feeding the west's appetite for illicit drugs. "We have gone through a tremendous experience – dramatic and costly for a society to live through. We have lost our best judges, our best politicians, our best journalists, our best policemen in this fight against drugs and the problem's still there."

It is difficult to overestimate the symbolic importance of a Colombian president entering the debate with such force, given the central role drugs have played in his country's recent bloody history. Santos is all too aware of the symbolism and of the role he is playing. "Yes, I know, and I'm conscious of what this means. I've told President Calderón [of Mexico], 'You and I have a lot more authority to talk about this because our countries have spilled a lot of blood fighting drug traffickers and we should promote this discussion."

If the war on drugs has failed, it has failed most abjectly in Latin America. That is where the bodies are buried. Or not so much buried, since the Mexican drug gangs prefer to litter the bodies of their victims along the byways and highways of the border towns with America, or leave them hanging from bridges to serve as a public warning to anyone who gets in their way.

Last week drugs gangs beheaded a blogger in Nuevo Laredo for reporting on the activities of the Zetas, the narcotics gang that all but controls the Mexican city that sits on its border with America. A month earlier, they beheaded a 39-year-old woman who blogged for the same site. In September, they hanged a couple from a highway overpass and left a note saying they had been killed for "their social media activity". These are four killings out of about 42,000 in the past five years. The price of drugs in Latin America can be costed in dollars, but in wasted lives too.

The fallout from the interminable war goes deeper – since the vast funds of narcotics trafficking have been used to corrupt their bodies politic. One former Colombian president, Ernesto Samper, has been publicly accused of having been swept to power on the back of the Cali drug cartels. Drugs have posed a threat to the very existence of civic institutions in many of the countries on the frontline of the war on drugs.

But Latin America is starting to take the fight to the consuming nations in Europe and the US. President Felipe Calderón of Mexico joined the debate in September when he used a speech in New York to hit out at consumer nations that were not doing enough to reduce demand. He took direct aim at the US, saying: "We are living in the same building. And our neighbour is the largest consumer of drugs in the world and everybody wants to sell him drugs through our doors and windows."

Calderón went further and suggested that if the consumption of drugs could not be limited, "then decision-makers must seek more solutions – including market alternatives – in order to reduce the astronomical earnings of criminal organisations". The phrase "market alternatives" was widely assumed to be a call for a new debate in the US about whether legalised or regulated drug markets might be an alternative to the war on drugs.

The more vociferous these Latin American voices become, the more difficult it will be for the leaders of the consuming nations to remain silent in the debate over the effectiveness of the war.

It was these western leaders that the Global Commission on Drug Policy was addressing when it released its landmark report earlier this year. The 19-person commission includes former UN secretary general Kofi Annan, former US secretary of state George Shultz, former US Federal Reserve chairman Paul Volcker and former presidents Ernesto Zedillo of Mexico, Fernando Henrique Cardoso of Brazil and César Gaviria of Colombia.

The report's first line was: "The war on drugs has failed." After detailing the costs, ineffectiveness and harmful effects of the drugs war, it made this plea: "Political leaders and public figures should have the courage to articulate publicly what many of them acknowledge privately… that the war on drugs has not, and cannot, be won."

This week a House of Lords event on drugs policy reform, organised by Baroness Meacher, will include an impressive list of attendees from around the world. It is an attempt to engage the debate, but no frontline British politicians will be there to hear people such as the Colombian interior minister speak. Privately, many senior British politicians support the initiative to try to help generate a new debate on drugs – but publicly they are invisible.

So it is left to Santos and others to stir the debate and try to promote a wider discussion. "I hope there is a shift in the debate. I am open to, and I welcome these discussions and this debate," he says. "We are the country who has suffered most of any country. Hopefully the world will enter into a fruitful and dynamic debate on this issue and if they find a new solution I'll be even more than happy to support it."

But political leaders in the consuming countries have not yet shown any appetite for joining the debate. In fact, quite the opposite. "This is a very sensitive political subject and there's a lot of hypocrisy there," says Santos. "Many leaders, in private, they will say something and they tell me something and in public they say, 'But I can't do this probably because my people will really crucify me'."

One of the most glaring contradictions is in the United States. While on the one hand a growing number of states in the US have semi-legalised marijuana (it is freely available from cannabis dispensaries with an easy-to-obtain doctor's prescription) on the other hand the country pours billions of dollars into helping the Mexican military fight the drug cartels which are busy trying to get marijuana into the US.

Barack Obama declared the war on drugs to be "an utter failure". He went on to say: "We need to rethink how we are operating in the drug wars because currently we are not doing a good job." But that was in January 2004.

There are, of course, isolated victories in the war and the manner in which Colombia disrupted much of the drug trade is a case in point. This was principally because of Plan Colombia, which involved a massive programme of financial and military aid. While Plan Colombia is credited with having saved the Colombian state, the "victory", as even Santos admits, is a Pyrrhic one.

"We are now helping other countries, the Caribbean countries, Central American countries, Mexico, because our success means more problems for them," he says. "There's the balloon effect." Meaning, that the problem is simply displaced, to another country – or even another continent, as in the case of Guinea Bissau in west Africa.

The other indices of the war on drugs do not make for encouraging reading. The National Survey on Drug Use and Health estimates that nearly 23 million Americans are illicit drug users. That is 8.9% of its adult population, up from 2008-09 when the rate was 8%. The number of marijuana users has gone up from 14.4 million in 2007 to 17.4 million in 2010.

As for the amount of land given over to the planting of coca – the raw material of cocaine – estimates vary. The UN suggests that coca production has fallen in Colombia – but neighbouring countries have seen a rise. The balloon effect plays out here too – if planting and transporting are disrupted in one part of Latin America the problem goes away – to a neighbouring country.

Santos is sketching a new future for Colombia and trying to imagine one that will not involve his country being held back by either narcotics or guerrilla warfare. His military attacks on the Farc go hand in hand with a determined attempt to try to wipe out the country's extreme poverty – the social and economic malaise which first brought the guerrilla group into life.

By Colombia's own reckoning, there are up to seven million people living in extreme poverty (favela-like housing with no electricity or clean water). Santos says: "We want to be a country with a competitive edge in the world. And a country with a solid democracy. To do that we need to attack the social problems, and extreme poverty is probably the worst of those. People in the UK don't imagine what it is to live in extreme poverty here in Colombia or anywhere in the so-called third world."

"I think that we are trying to move towards the first world slowly but surely. But we must do a good job for the people left way behind. That's why extreme poverty for us is a priority. There was a phrase that President Kennedy used to use a lot, 'You cannot be rich if you're surrounded by poor'. And Colombia is a very unequal country, one of the most unequal countries in the world. If we don't correct that we will never be really competitive and we will never really have a solid democracy."

His impressive poverty tsar, Samuel Azout, a former businessman and philanthropist, is leading the drive to eradicate extreme poverty. In his office there is a framed portrait of Kennedy and a series of large framed signs. One reads: "A business that only makes money is a poor business." Another says: "The causes of poverty are interconnected, so the solutions should be joined up too: health, education, housing, justice."

"Less poverty aids economic growth," says Azout. "Economic growth is necessary but not sufficient in lifting people out of poverty. You also need direct action. Extreme poverty is an obsession for me, and for President Santos too." A hugely ambitious programme launched last week involves housing, child development centres, social workers and the establishment of "extreme poverty-free zones".

Perhaps even more significant for his country is the law Santos passed in July, the victims and land restitution law – an attempt finally to restore millions of acres of land to Colombians driven from their homes by the decades of violence. Many Colombian observers feel that this will be Santos's legacy. In the past 20 years, nearly 4 million people across an area of 6.5m hectares (16m acres) have been displaced by armed conflict.

Santos says: "In 10 years' time I hope that people will say finally we are a country that is living in peace and that we have a very strong democracy, a dynamic democracy that has been able to progress socially and that we no longer have this shameful title of being one of the most unequal countries in Latin America."

When Santos arrives in London, it will be to sell the new Colombia and help to drive British investment, which he sees as potentially a key player in his country's development. He is not likely to spend much time talking about drugs, but he has this message for young Britons: "I will say to them that, besides the blood that every sniff of cocaine produces, it's also producing something to which the UK youth and the European youth and the youth around the world are more and more sensitive. It's creating havoc to the environment. Cocaine is probably the worst enemy of tropical forest. Much of the deforestation that you see in Colombia, in Peru, in Brazil is because of cocaine production. So it is not only the blood that it creates, the violence it creates – it's destroying the world."