The sale Thursday comes two months after Corporación Nacional del Cobre de Chile, or Codelco, announced plans to exercise an option to buy a 49% stake, backed by a $6.75 billion bridge loan. Under terms of the option agreement, Codelco's claim on Anglo American Sur is reduced by the percentage stake that is not owned by Anglo American. The sale to Mitsubishi leaves the Chilean group with the right to buy just a 24.5% stake in Anglo American Sur, according to Anglo American.

"There was very clearly a value gap," said Anglo American spokesman James Wyatt-Tilby, who added his company expected to receive only about $6 billion from Codelco for the 49% holding. "Anglo American has acted in the best interest of its shareholders and will continue to do so."

London-listed Anglo American will consider selling a further stake in Anglo American Sur if Codelco is not willing to make an offer for the remaining stake at a price above the value stipulated in the option agreement. If Anglo American sold a further 24.5% stake it believes that Codelco's claim on Anglo American Sur's equity would be cut to zero.

Anglo American has received approaches regarding the acquisition of a stake in Anglo American Sur from several parties, CEO Cynthia Carroll told journalists on a conference call.

Anglo American and Codelco have been talking for months about the prospect of the Chilean group exercising its option and Anglo earlier this year offered to buy Codelco out of the agreement.

Codelco, led by Diego Hernández, said that it would contest the sale to Mitsubishi, adding that the deal did not affect its claim on a 49% stake in Anglo American Sur. Codelco will "exercise all the necessary actions to wholly reaffirm its rights," the Chilean group said.

Mitsubishi's acquisition values Anglo American Sur at $22 billion, equal to about 17 times the $1.3 billion Ebitda that the unit posted in 2010. Anglo American Sur's assets include the Los Bronces mine, the open pit El Soldado mine and the Chagres smelter, all of which are in Chile. The operations have proven and probable reserves of 2.4 billion tons and additional resources of about 6.4 billion tons.

Codelco's option to buy a stake in Anglo American Sur is a legacy of an agreement struck between the operation's former owner Exxon Mobil Corp. and Empresa Nacional de Minería, a Chilean state-owned mining company. Anglo American bought the mine from Exxon in 2002.

Codelco in 2008 bought the option, which is available to be exercised in January once every three years. The state-owned copper producer said Oct. 28 that it planned to exercise the option and had secured $6.75 million in financing from Japan's Mitsui & Co. Ltd.

"The sale of an interest in AAS demonstrates the board's commitment to delivering value for our shareholders, in line with the contractual alternatives available to us in our agreement with Codelco," Anglo American chairman John Parker said in a statement.

Anglo American took financial advice on the deal from UBS and Goldman Sachs Group Inc. and legal counsel from Shearman & Sterling LLP's George Casey, Michael McGuinness and Rory O'Halloran.