The decision is further evidence that Xstrata's board, which supports both the deal and the bonuses, could yet face a significant snub when shareholders vote on the deal on Nov. 20.

Xstrata took out advertisements over the weekend asking shareholders to back the £22 billion ($35 billion) bid for the 66% of Xstrata not already owned by Glencore and to support plans to pay Xstrata management about £144 million of retention bonuses.

"It is unusual to see companies advertising for shareholders to vote but I think the Xstrata management are a bit worried about their incentive package being voted down and there are scenarios where that could scupper the deal," David Cummings, head of U.K. equities at Standard Life Investments, said in a Monday morning radio interview with British Broadcasting Corp. "I do think that the incentive package should be voted down, I think it's unnecessary and, to be honest, I think it's a combination of a rather rapacious management team and a weak board, and we will be voting against it."

Standard Life's position on the deal is further evidence of a split in voting intentions amongst Xstrata shareholders. Its position mirrors that of shareholder advisory groups Institutional Shareholder Services Inc. and the Association of British Insurers, which both recommended voting for the deal but against the bonuses.

Scottish Widows Investment Partnership, which is also a top ten shareholder in Xstrata, said last week that it will back the deal and the bonuses. Others, including Schroder Investment Management Ltd., Knight Vinke Asset Management LLC and British pension fund adviser Pensions Investment Research Consultants Ltd. have come out against the deal and the bonuses.

Xstrata shareholders have the option to vote against Glencore's offer of 3.05 shares for each Xstrata share, for the offer and the bonuses or for the offer and against the bonuses.