* Tie-up approved by 99.9 percent of votes cast
* Eyes operating marging of 40 percent in medium term
* Values Gartmore at 391.4 million pounds (Updates with result of vote)
LONDON, March 22 (Reuters) - Henderson shareholders have voted overwhelmingly in favour of a takeover of rival fund manager Gartmore, in a vote at the company's general meeting on Tuesday.
A total of 432.11 million votes were cast in support of the acquisition, equivalent to 99.9 percent of votes cast, Henderson said in a statement.
Votes against amounted to 97,157, and just over 1 million votes were withheld.
Shareholders would see the benefits of taking over rival fund manager Gartmore from this year, Henderson's chairman had told shareholders earlier on Tuesday as they prepared to vote on the deal.
"It will generate a significant enhancement in underlying earnings per share, and underlying returns are expected to exceed our cost of capital from 2011, in each case before integration and deal costs," Rupert Pennant-Rea told shareholders.
The acquisition, valuing Gartmore at 391.4 million pounds ($637 million), will "fill gaps" in Henderson's investment capabilities, especially global equities and emerging markets equities, said the chairman.
CEO Andrew Formica said last month the deal would allow the new fund manager to focus on high-fee management products.
Henderson has said it is retaining Gartmore fund managers in charge of 84 percent of the fund manager's assets under management, accounting for 90 percent of the revenues.
The intended merger will result in a combined operating margin of 35 percent, "with a medium-term aim of closer to 40 percent" the chairman said.
"Clearly the financial benefits of this transaction are compelling," he said. ($1=.6143 Pound) (Reporting by Cecilia Valente, Editing by Sinead Cruise and Will Waterman)