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The top boss of a proposed Alaskan mine, which has been through a roller coaster of regulations for the past 15 years, called the recommendation that the US environmental agency should veto the project a “massive regulatory overreach”.

Mining waste from the Pebble Mine project, which has one of the world’s largest copper and gold deposits, would threaten Alaska’s Bristol Bay watershed, home to important salmon species including the world’s largest sockeye salmon fisheries that have supported critical wildlife and a multibillion-dollar industry, according to the Environmental Protection Agency (EPA).

Late Thursday, the EPA’s Pacific Northwest region, which includes Alaska, recommended the project should be vetoed.

In response, Pebble Limited Partnership chief executive John Shively on Friday called the EPA’s suggestion “wildly speculative” and “not supported by any defensible data”, adding the agency was acting outside the authority given to it by Congress.

The EPA did not immediately respond to a Reuters request for comment.

Pebble Limited Partnership is the main subsidiary of Canada-based Northern Dynasty Minerals.