It was the “clash of the titans” yesterday, as two powerful corporate boards squared off in another round of a public battle for control of Bermuda-based Aspen Insurance Holdings Limited.

The battle royal began anew as Aspen sent new filings to the SEC and its besieged board struck out at would-be suitor, Endurance Specialty Holdings Ltd., urging shareholders to resist Endurance’s “unsolicited” exchange offer.

Endurance is seeking to acquire all of the outstanding shares of Aspen for a combination of its common stock and cash. In a statement, the Aspen board; recommended shareholders reject “coercive Endurance tactics” to force through an “ill-conceived offer that significantly undervalues” Aspen; and issued an investor presentation (http://www.aspen.co/Investors-Media/Investor-Relations/Presentations/) detailing a stand-alone strategy for superior value creation.

But not to be outdone, Endurance responded to what they called “Aspen’s combative position”, saying it confirmed the need for Endurance proposals to give Aspen shareholders a voice in the conflict “to urge Aspen to come to the negotiating table, and to hold the Aspen board accountable for its actions.”

Aspen said that after careful consideration and discussions with its financial and legal advisers, it unanimously agreed to reject the Endurance offer, saying it was not in the best interests of the company, nor its shareholders.

The Aspen statement said the investor presentation details “the company’s strong stand-alone growth strategy, clear path for continued improvement in return on equity, and the inadequacy of Endurance’s offer.”

Glyn Jones, Aspen board chairman, said, “The Aspen board of directors is unanimous in its belief that the Endurance offer significantly undervalues Aspen and fails to reflect the value of our business and strong future prospects. We are highly confident that Aspen can achieve more value for its shareholders — and without the significant risks that are inherent in a merger with Endurance — by continuing to execute its strategic business plan.”

Mr. Jones added, “Beyond the offer’s significant undervaluation of our company, we believe that there is a fundamental strategic mismatch between Aspen and Endurance and that a combination would create significant dis-synergies. Additionally, the 60 percent stock component of Endurance’s offer is highly unappealing given Endurance’s unattractive business mix, with an overreliance on the volatile, low-margin and challenged crop insurance business and a dependency on reserve releases to fuel earnings. We urge shareholders not to tender their shares into Endurance’s offer.”

Aspen also announced that it has filed a preliminary revocation solicitation statement with the US. Securities and Exchange Commission (SEC) in connection with Endurance’s plan to seek support from Aspen shareholders to: force a special meeting of shareholders to vote to increase the size of Aspen’s board; and, its plan to seek a Bermuda Supreme Court order forcing a shareholder meeting to approve the Endurance scheme of arrangement.

Mr. Jones commented, “Both Endurance proposals are part and parcel of its desperate attempts to force through an inadequate offer for Aspen. We strongly urge shareholders not to support Endurance’s pursuit of an involuntary scheme of arrangement. If it proceeds, this convoluted legal manoeuvre could result in a ‘take-it or leave-it’ shareholder vote on a transaction under the terms currently proposed by Endurance. In addition, Endurance’s attempt to expand the Board, if successful, would result in an unwieldy and highly anomalous 19-member board ill-suited to protect the interests of Aspen shareholders. Our Board strongly believes that the continued execution of our strategic plan will generate significantly greater long-term value than Endurance’s offer.”

But Endurance came back with a statement to address “Aspen’s misleading assertions” and said that the offer provides compelling value to Aspen shareholders.

John R. Charman, Endurance’s Chairman and chief executive officer, said, “We know from our extensive engagement with Aspen shareholders that many have been frustrated by the dismissive and entrenched response of Aspen’s board and management to the significant opportunity for value creation that Endurance has proposed. The announcement made by Aspen today in response to our transaction proposal and exchange offer is simply more of the same — a self-interested cadre of Aspen insiders taking every action in their power to deny their own shareholders, the true owners of the company, the opportunity to realise the compelling value we have proposed.

“Perhaps it should come as no surprise that Aspen’s board and management are not aligned with the best interests of their shareholders given they collectively own less than 1.2 percent of Aspen,” Mr. Charman continued. “Despite the heated rhetoric Aspen has directed at Endurance, Aspen’s board and management have presented no credible plan to deliver value that can compete with what we are offering, which represents a 19.5 percent premium over Aspen’s previous all-time high share price and at a 1.16x multiple of March 31, 2014 book value is a 13.8 percent premium over Aspen’s highest previous book value multiple (1.02x) over the past five years. What Aspen’s board and management have failed to achieve for ten years, we are prepared to deliver today.

“We urge Aspen shareholders to speak forcefully by supporting our two shareholder proposals — calling for a special general meeting to increase the size of the Aspen board, which will lead to a majority of Aspen’s directors standing for election at its 2015 annual general meeting, and endorsing the pursuit of a court-sanctioned scheme of arrangement.

“We are truly committed to pursuing these and any other actions and are determined to see them through in order to provide Aspen shareholders the ability to realise the highly attractive premium our proposal represents,” Mr. Charman said.

“We firmly believe that a combination of Endurance and Aspen makes eminent strategic and financial sense, creating a company with increased scale, an attractive diversified platform across products and geographies, and greater market presence and relevance. Our proposal offers Aspen’s shareholders a highly attractive upfront premium and the ability to participate in a stronger and more profitable company. Aspen shareholders deserve a Board and management team that does not wilfully ignore the compelling benefits of our proposal and should use the opportunities Endurance has presented to them to make their opinions known on this value-enhancing combination,” Mr. Charman concluded.

Complete statements from both companies can be found on their corporate websites.