Davis Selected Advisers, which owns 23.6 percent of Transatlantic, previously said it might oppose the deal, which was announced in June.
“Davis Advisers believes that the current offer from Allied World Assurance Company Holdings is not in the best interest of maximizing value for shareholders,” Davis said in a filing with the U.S. Securities and Exchange Commission Wednesday.
Because of an agreement with New York insurance regulators, the firm can only vote a 9.9 percent stake freely and has to vote its remaining shares in proportion to the votes of other shareholders.
An Allied World Assurance Company Holdings Inc spokeswoman could not immediately comment on the filing. A Transatlantic spokesman declined to comment.
Davis has not said anything publicly about the other two bids — a hostile offer from Validus Holdings Ltd and an unsolicited offer from Berkshire Hathaway Inc.
Validus has taken its offer directly to shareholders, while Transatlantic and Berkshire are in negotiations.
At Wednesday’s closing prices, the Allied World all-stock offer was worth $2.9 billion, while the Validus cash-and-stock offer was worth $2.99 billion and the Berkshire Hathaway all-cash offer was worth $3.25 billion.
The deal has become very contentious, with Transatlantic and Validus suing each other and Transatlantic shareholders suing the company as well.