The landowning group at the site of Bougainville's Panguna mine says it has won the backing of one of the formerly militant Me'ekamui groups.
The Special Mining Lease Osikaiang Landowners Association wants to team up with Australian company RTG to re-open the mine.
But the government in the autonomous Papua New Guinea region wants to work with another Australian company, Caballus, and to change the mining law to do so.
Osikaiang's Philip Miriori said his group has been joined in its opposition to the law change by ex-combatants, led by former military commanders Moses Pipiro and Peter Onabui.
He said they back Osikaiang in opposing any involvement from Caballus or from Bougainville Copper Ltd, the company that operated the mine before it was forced to close by the civil war.
There is a strong groundswell at Panguna in opposition to the Bougainville government's plans to change the Mining Act, Mr Miriori said.
In a statement, he quoted Mr Pipiro saying "they must stop playing games with the future of Bougainville and stop the theft of our mineral rights and ownership immediately".