Legalisation of cannabis is making slow but unstoppable progress across much of the developed world, many experts believe, following the end of prohibition in two US states.

In Amsterdam, long famous for its coffee shops, identifiable by pictures of marijuana outside and fumes wafting through the door, international experts gathering to discuss cannabis regulation said the international conventions, once so heavily policed by the US, would now be increasingly flouted. Already many countries, most notably the Netherlands and Spain, have bypassed the rules.

"The key is always the US in drug policy," said Professor Robin Room of the charity Turning Point and Melbourne University in Australia. "It would be acting differently if it did not have Colorado and Washington on its conscience.

"The dynamic in the United States looks unstoppable. Even if a Republican were elected to replace Obama, they would think twice before cracking down on it all. What would they gain from it? The Republicans realise they are in a box, appealing only to elderly white men.

"It is an ocean liner that is exceedingly difficult to turn around but there will probably be a state or two in 2014 and another couple in 2016 voting for legalisation. That poses a big conundrum to the world: the most important nation in drugs policy is in defiance of the treaty it was responsible for."

The position of the US as the world's enforcer of drugs prohibition has been undermined not only by cannabis legalisation in Colorado and Washington states, but also Uruguay in South America.

The Obama administration has elected to ignore any flouting of the UN drug conventions of 1961, 1971 and 1988, which require countries to prohibit the use, possession and supply of cannabis, although the US justice department has warned against allowing children to obtain the drug and profits going to criminal gangs.

The treaty is rarely observed but countries evade it in different ways. In Amsterdam and many, but not all, places in the Netherlands, the sale and use of cannabis is tolerated but not legal. But supply is illegal and prosecuted, which has led to the serious problems that have caused some towns to shut their coffee shops.

Drug gangs involved in all kinds of criminality, including trafficking, flourish by supplying the drug. The Netherlands is deeply distressed that toleration may be encouraging crime, to the point where its justice minister favours closing down the cannabis trade altogether. But the model has spread. There are coffee shops in Ljubljana, Zagreb and Berlin.

Spain, on the other hand, has networks of "cannabis social clubs", where people get together to grow what they need for personal use. In the UK, now one of the countries with the toughest stance on cannabis, social clubs on the Spanish model have been quietly taking off. There are now 60 of them, with some 60,000 people in some way involved, said Greg de Hoedt, who is head of the UK Cannabis Social Clubs, a body set up to advise them and advocate for change and which has been busy lobbying politicians. Nick Clegg is one of those onside, said De Hoedt, who was off to meet the coalition Home Office minister Norman Baker.

Clubs secretly grow their own cannabis. "It's about 100 times less than it costs on the street and it's a better quality product," said De Hoedt, who complains that the drug available on the street can contain fertilisers and sometimes heavy metals. "Criminal gangs aren't taking account of that," he said.

A woman smokes cannabis at a '420 Day' demonstration in London's Hyde Park, calling for cannabis to be legalised. Photograph: Andy Rain/Camera Press

The clubs also organise collective acts of civil disobedience. On 20 April – now a global celebration of cannabis known as "420 day" – about 10,000 people gathered in Hyde Park to smoke joints in defiance of the police. "People lit up en masse at 4.20[pm]," said De Hoedt. There were a few arrests and cautions, but that was all. De Hoedt said he understood the concern of the police that drug gangs might turn up, which is why the law needed to change.

The Amsterdam meeting was convened by academics and public health experts involved in a five-year research project, funded by the European commission, called Alice Rap (Addiction and Lifestyles in Contemporary Europe Reframing Addictions Project). It launched a policy brief unequivocally backing the legal regulation of cannabis. It rehearses the economic arguments – the costs of pursuing cannabis crimes versus the tax revenue from legal production – and the health arguments around separating cannabis from other drugs and checking on quality and strength. But the discussion was around not whether but how it should happen.

"We are talking about a drug that is still problematic but by prohibition you make the problem bigger than it is," said Dr Franz Trautmann of the Trimbos Institute – the Netherlands institute of mental health and addiction.

The prevalence of cannabis in Europe is between 5% and 20%, he said, but appeared not to be affected by drug policy. Spain, France and Britain all have higher than average rates in Europe.

"We have to deal with and acknowledge it is used by so many people," he said, adding that a market existed that needed to be regulated. "It touches security, public order and safety in society. We have created a situation which has contributed to organised crime.

"Cannabis is moving from prohibition to regulation. Tobacco, alcohol, gambling are moving from uncontrolled to regulation. If you see how tobacco or alcohol is regulated, it is similar to what we do with cannabis [no under 18s in coffee shops, not near schools, staff do not work in smoking rooms etc]. We know these measures are effective for tobacco. We are not able to ban the drug totally but we can control it better."

A lone voice against regulation was that of Dr Susan Weiss, of the National Institute on Drug Abuse (NIDA)in the USA, who has serious concerns about Colorado, which has ads for chocolate spread with "medical marijuana" and a free joint with a ski pass.

"Things are moving very quickly and I'm afraid they are going to move even faster," she said. "This is an issue that a lot of states are considering. Part of it is because they see all the tax revenue that could be generated. One of the concerns I have is that it could be shortsighted because they could be missing all the health harms.

"We have this idea that we know exactly what cannabis does because it has been around such a long time. But I don't think we have a full understanding."

She is worried too about the effect on young brains and the combination of marijuana and alcohol, which NIDA is studying.

Some experts think the international conventions will just wither away. Few think there is a chance of overturning them. Russia, China, Sweden, Japan and parts of Africa are among the nations that would block it.

Trautmann says it is not good to ignore international conventions, "but the more initiatives we get, the more potential we get to show that the regulations are not working and the more motivation and support we get to change something."