Zions Bancorp. and First Security Corp. late Monday disclosed a previously unnoticed incentive plan provision that, based on First Security's current share price, will trigger a multimillion-dollar increase in charges related to the banking companies' planned merger.

The clause in First Security management's stock incentive plans will accelerate the vesting of stock options for a brief period after shareholders vote on the merger, which was announced June 6. Because of this, accounting rules require the value of these options to be recognized in First Security's income statement as a compensation charge, to be added to the $210 million merger-related charge previously announced by the banking companies.

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