A majority of Americans would, for the first time, support the legalisation of marijuana, the Gallup organisation reports, suggesting a cultural shift across the country on a scale similar to that seen on gay marriage.

Taken nearly a year after voters in Colorado and Washington broke new ground by voting to legalise the sale of the drug, the survey indicates a change in attitudes that is swift and significant. A full 58 per cent of respondents said they would favour decriminalisation, compared with 50 per cent in 2011. When Gallup first posed the question in 1969 – during the blossoming of “flower power” – only 12 per cent supported it.

Just like gay marriage, now legal in 14 states, the decriminalisation movement has plenty of space to grow. Last week, the second highest elected official in California, Lieutenant Governor Gavin Newsom, said he was ready to legalise the drug in the state and advocates are preparing to put the issue to Californian voters next year. The question is also being debated in other states and even some municipalities, such as Portland in Maine.

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Some impetus may even have come from the federal government. Even though it still lists marijuana as a prohibited substance, in August the Justice Department informed Washington and Colorado that it would not intervene as they prepare for the first marijuana cash registers to start ringing in the new year.

“Americans are increasingly recognising that marijuana is less harmful than they’ve been led to believe,” Mason Tvert, a spokesman for Marijuana Policy Project, said in response to the poll. “I think it’s time to regulate marijuana like alcohol and most Americans appear to agree. We’re seeing support for ending prohibition in states across the country and efforts are being made to change state laws.”

Not surprisingly, the Gallup poll shows strongest support among younger Americans and those who call themselves political independents.

“Whatever the reasons for Americans’ greater acceptance of marijuana, it is likely that this momentum will spur further legalisation efforts across the United States,” said Art Swift, Gallup’s managing editor.

While personal consumption may be one factor – answering a different survey in August 38 per cent of Americans admitted to having tried the drug – there is also the increasing popularity of marijuana for medical use. “The increasing prevalence of medical marijuana as a socially acceptable way to alleviate symptoms of diseases such as arthritis, and as a way to mitigate side effects of chemotherapy, may have also contributed,” Mr Swift suggested.

While Washington’s hands-off approach so far has been welcomed by Colorado and Washington it has angered others. This week, the Attorney General, Eric Holder, suffered a public scolding at the annual meeting of the International Association of Police Chiefs in Philadelphia.

“We are disappointed with the decision,” said the group’s president, Craig Steckler. “We think we’ve opened the floodgates to people who want to fully legalise all drugs.” America, he said, was on “slippery slope”.

Both states have spent the last 12 months putting in place a series of controls to regulate the sale of the drug. Washington is preparing to approve the opening of more than 300 approved retail “pot shops”.

The release of the Gallup poll meanwhile coincided with the opening in Denver, Colorado, of the 2013 International Drug Policy Reform Conference, to be attended by more than a thousand experts in related areas of law enforcement, treatment and healthcare. Also on hand in Denver will be top government officials from Uruguay which is in the middle of planning legalised sales of marijuana.

The impact of the debate on national politics in the United States is not yet clear. Chris Christie, the Governor of New Jersey and a likely Republican presidential runner in 2016, has signed a bill increasing access to medical marijuana, including for children.

And Matt Yglesias, economics writer for the online Slate magazine, tweeted: “I forecast high odds that Hillary Clinton will “evolve” on marijuana prohibition in the next couple of years.”