Henderson's Stephen Peak complained bitterly about the £1.2m discretionary bonus paid to Gartmore CEO Jeff Meyer and demanded it be revoked.
Peak, who owned around 14% in Gartmore across a range of funds, says he was "staggered" and "incandescent" to learn of Meyer's bonus and described it as "a significant payment for ultimate failure", the Financial Mail reports.
When deciding to grant Meyer a bonus, chairman Andrew Skirton and Gartmore's other non-executive directors "lost the plot" and "should hang their heads in shame", Peak said.
After the scale of Meyer's bonus was revealed last month, the fund manager wrote to Skirton to voice his anger but he refused to cancel the payment, the Financial Mail reports.
He replied to Peak that Meyer earned the bonus by readying Gartmore for sale.
Peak has now written to Meyer directly, asking him to decline the money or donate it to charity but he has so far not received a reply.
Meyer's £1.2m bonus was part of a wider package amounting to almost £10m paid after a period in which Gartmore halved in value before being sold to Henderson for £336m in January.
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