A leaked British report into the archaeological significance of an ancient Roman gold mine has helped to scupper plans by the Romanian government to approve invasive mining at the site.

The expert report, kept hidden for three years by the Bucharest government, was commissioned by Romania's ministry of culture and funded by a not-for-profit organisation, Pro Patrimonio, which works to protect Romania's cultural heritage. The report says that the ancient site, in Rosia Montana in the Apuseni Mountains of western Transylvania, is worthy of consideration as a Unesco world heritage site and that its galleries are "the most extensive and most important underground Roman gold mine known anywhere".

This month, the ministry of culture presented a list of monuments that it would like to see included as world heritage sites, but the picturesque village of Rosia Montana, with its ancient galleries that tell of Roman mining, was not on it.

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The village sits near one of the largest undeveloped European gold deposits. A Canadian mining company, Gabriel Resources, wants to extract the gold using a method that would reportedly require 40 tons of cyanide daily. Politicians have backed the plans.

In 2010, the town's mayor, Eugen Furdui, admitted: "If Rosia Montana were added to the Unesco world heritage list, that would automatically mean that mining [could not] go through. And we want this mining project to be carried on."

The report's authors – Andrew Wilson and David Mattingly, professors of Roman archaeology at Oxford University and Leicester University respectively, and Mike Dawson, director of archaeology at the environmental consultancy firm CgMs – travelled to the site and were impressed by what they found.

"The key thing we were asked to do was to evaluate the site and see if it was a worthy consideration to be a Unesco world heritage site," Professor Dawson said. "Our opinion is that it has a very high status."

The experts' findings remained unknown until November, when the report was leaked, backing up hundreds of thousands of protesters who had been pressing the Romanian government to drop legislation that would have enabled Gabriel Resources' project to go ahead – destroying villages and mountains.

"I am glad the public [can now see] that information," Professor Dawson said. "It deserves to be out there."

A government commission has now rejected the mining proposal. However, opponents warn that the reprieve is only temporary, as the rejection was based on technical and legislative reasons rather than environmental.

This means that the door remains open for companies such as Gabriel Resources to submit revised proposals. In response to the latest rejection, Jonathan Henry, chief executive of Gabriel Resources, said: "Our goal remains to bring the project through to a reality that will significantly benefit Romania and Rosia Montana."

With the leaking of the British study, it will be difficult for the Romanian government to deny the contribution that the area makes to world culture. But, as Professor Dawson warned, the financial gains from mining are high.

"In my experience, money talks," he said. "If [the report] is criticised, it will be criticised on the basis that conservation costs money."