The state of Washington could raise at least an extra $560m (£354m) in new taxes if voters pass Initiative 502 to legalise marijuana in November, a report says.

The state's Office of Financial Management (OFM) issued a preliminary fiscal note on the financial implications of voters passing I-502. The note claims that approving the initiative will generate an annual revenue of $564m-$606m, at least $130m more a year in taxes than the current state alcohol monopoly and more than double what the Initiative 502 campaign itself had predicted.

Under I-502, the sale of 28 grams (one ounce) of marijuana to people over the age of 21 from state-licensed shops would be strictly regulated and taxed.

The OFM estimated that most of the revenue - more than $433m - would be generated by a 25 percent surcharge that would appear at each step from grower to retailer. Business and sales taxes would raise an estimated $130 million more.

The OFM's fiscal note arrived at these figures by assuming "retailers will sell 85.1 to 93.6 million grams per year at an average price, excluding taxes, of $12 per gram".

In an email to the IB Times UK, OFM communications director Ralph Thomas explained how the figures were reached, including the retail price and how much will be sold in the first year.

"Medical marijuana dispensary prices on average range between $10 and $15 per gram with some premium products exceeding $15 per gram. Based on average retail mark-up practices, producer price is $3 per gram and processor price is $6 per gram," Thomas said.

But this analysis is only a guesstimate of what a legal marijuana industry might look like. It estimates 100 state-licensed growers supplying more than 300 marijuana stores that - based on federal drug-use surveys - would sell 187,666 pounds to at least 363,000 customers.

Alison Holcomb, campaign director for pro-I-502 group New Approach Washington has welcomed the preliminary fiscal note. She said: "The fiscal note helps voters see, in concrete terms, how current marijuana policy is depriving our communities of resources for vital government services and simply funnelling those funds into the black market."

I-502 has started to draw national attention in the United States not only because it would be the first stepping stone to legalising marijuana in the US if passed. It also has some high-profile supporters, including Charles Mandigo, the FBI's former top agent in Seattle, as well as former US attorneys John McKay and Katrina C Pflaumer.

It has also gained support from Law Enforcement Against Prohibition (LEAP), a group of police officers who would like to see marijuana legalised and regulated.

Norm Stamper, former Seattle chief and a spokesman for the group, said: "Everyone knows that marijuana prohibition has failed.

"When even those who once worked to enforce these laws are saying this, the only logical next step is to enact a system that legalises, regulates and controls marijuana.

"Doing so will not only take money away from the gangs and cartels that sell marijuana now, but will generate new, much-needed revenue that can be used to pay the salaries of police officers and teachers and for substance abuse prevention and education."

James Doherty, a former prosecutor in Seattle, added: "By regulating and controlling marijuana, we will make it less available to teenagers. Ask any high school student whether it is easier to get marijuana or alcohol. Most will say marijuana, because alcohol is regulated and controlled under the law and marijuana is controlled by illegal dealers who don't ask for ID."