World no.2 iron ore miner Rio Tinto lost a court fight on Friday over an old non-union agreement at its highly profitable iron ore mines, ending a three-year-old battle rendered moot by changes in Australian industrial relations laws.

The High Court of Australia on Friday refused a request by Rio Tinto to appeal a ruling by the Federal Court which established that a 2008 agreement Rio Tinto had with its Pilbara iron ore miners in Western Australia, bypassing unions, was not valid.

Miners have shunned union representation for many years in the Pilbara mines, including rival BHP Billiton's mines, even though unions have been free to recruit members since the Labour government put in new workplace laws in 2010.

Rio Tinto's head of Pilbara operations, Greg Lilleyman, told workers that the High Court's decision would not change anything for workers who were covered by the 2008 agreement and those who were not.

As has been the case, since the decision was made last year, employees that were covered by the PIEA (Pilbara Iron Ore Employees Agreement) will continue to receive the same pay and conditions as they had before, Lilleyman said in a memo obtained by Reuters.

The Labour government overhauled industrial relations laws, giving unions more power to bargain, nevertheless the unions have not had much influence in Western Australia iron ore mines.

Chief Executive Tom Albanese said on Thursday he was worried about escalating labour costs in Australia, now one of the most expensive places the company is operating.

Throughout Australia we've been impacted by rising costs and what I'm concerned about is declining productivity, he told reporters after the company reported a 6 percent drop in underlying earnings for July-December.

BHP Billiton is facing a one-week strike next week at its Bowen Basin coal mines in Australia, where CEO Marius Kloppers said this week union demands go beyond wages and conditions and will affect the company's expansion plans.

The Construction, Forestry, Mining and Energy Union (CFMEU), which brought the case against Rio Tinto, welcomed the decision.

This is potentially quite significant for the unions, Andrew Vickers, general secretary of the CFMEU, told Reuters.

He said it would take some time for workers in the Pilbara to digest the ruling and decide whether they wanted to join the union and let the union represent them in bargaining with Rio Tinto.

(Reporting by Sonali Paul; Editing by Richard Pullin)