Institutional Shareholder Services, the biggest proxy advisory firm, wrote in a report that Transatlantic shareholders could get more money, given higher competing bids from Validus Holdings and Berkshire Hathaway.

Transatlantic shareholders are scheduled to vote on the Allied deal on Sept. 20.

The Transatlantic-Allied deal has faced hurdles since it was announced in June. Late last month, Transatlantic’s biggest shareholder, Davis Selected Advisers, publicly opposed the Allied deal. Davis’s holdings represent 9.9 percent of Transatlantic’s voting shares.

I.S.S.’s report bolsters arguments by Validus, another insurance firm that is pursuing a $2.89 billion hostile bid for Transatlantic. Validus has argued that its stock-and-cash bid offers the most for Transatlantic shareholders, and contended that its target is improperly refusing to begin merger negotiations.

Validus has also said that it might offer a higher price if Transatlantic were to begin talks.

Transatlantic has also been talking with a unit of Berkshire Hathaway, which bid $52 a share, in hopes of securing a higher bid.

The value of both the Validus and Allied offers has steadily fallen since each was announced. As of Friday’s market close, Validus’s offer was worth $46.27 a share, while Allied’s was valued at $45.70 a share. Transatlantic’s stock closed at $48.83.

Transatlantic and Allied have argued that their merger makes the most sense, creating a firm that offers both reinsurance and specialty insurance like environmental liability policies.

A spokesman for Transatlantic said in a statement: “We are disappointed with the recommendation because a merger with Allied World would accelerate our ability to accomplish our strategic objectives.”

A spokesman for Allied declined to comment.

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