The United Steelworkers union, in a letter to investors of Grupo Mexico SA de CV and its Southern Copper Corp. (SCC) subsidiary, alleges that the companies' mismanagement of labor relations has cost shareholders more than $2 billion in operating income.

By the union's calculations, if ongoing strikes at Grupo Mexico's Cananea open-pit copper mine, Taxco silver-zinc-lead mine and San Martin copper-zinc mine persist through 2009, the company will have lost $2.3 billion in earnings before interest and taxes (EBIT), as well as 1 billion...