South African metal workers at BHP Billiton Ltd.'s (BHP) Meyerton alloys operation will put down their tools Thursday to put pressure on the company to raise wages, their union said.

The National Union of Metalworkers said it has received a certificate allowing its members to begin a "protracted and militant strike action."

Numsa is demanding an 8.8% increase in pay for its members, but said BHP Billiton has only offered 2%.

"The failure by BHP Billiton to adhere to workers demands, Numsa will be left with no option but call on other Numsa members in various plants at BHP Billiton to strike in solidarity with the Meyerton workers," the union said in an e-mailed statement.

A spokesman for Numsa wasn't immediately able to say how many members the union represents at Meyerton.

-By Robb M. Stewart, Dow Jones Newswires; +27 11 783 7848; robb.stewart@dowjones.com