It may be a case of now or never for Aspen Insurance Holdings shareholders considering whether or not they should support Endurance Specialty Holdings’ hostile takeover bid.

Yesterday, Endurance chairman and chief executive officer John Charman said in a statement that Aspen shareholders “risk losing all of the immediate benefits and opportunity for future value creation” of the proposed merger if they vote against Endurance’s proposals to expand the size of the Aspen board in order to smooth the way to a takeover.

Mr Charman’s comments came after proxy advisory service Institutional Shareholder Services (ISS) advised against supporting Endurance’s two shareholder proposals.

Both Aspen and Endurance have vigorously lobbied Aspen shareholders.

Endurance made a cash-and-stock offer for its Bermuda re/insurance rival of $49.50 per share on June 2, worth around $3.2 billion. The Aspen board firmly rejected Endurance’s offer after which Endurance made its proposals to expand the Aspen board.

“The ability to demand good corporate governance and effect change at Aspen is now in the hands of Aspen shareholders,” Mr Charman said in the statement yesterday. “We strongly encourage Aspen shareholders to vote for our shareholder proposals or risk losing all of the immediate benefits and opportunity for future value creation of our proposed transaction.”

Mr Charman made it clear that Endurance was at odds with the conclusion of the ISS report.

“While we are disappointed by the ISS recommendations and strongly disagree with the positions taken, Aspen shareholders should note that the ISS report explicitly takes no position whatsoever on the merits of our offer and surprisingly ignores the substantial and fundamental issues of poor corporate governance continuously demonstrated by Aspen’s board and management, relying instead on questionable technical assessments of vote timing and mechanics. “ISS fails to recognise that Endurance was forced to take the two proposals directly to Aspen shareholders because of the failure of Aspen’s board and management to enter into discussions with Endurance.”

He added that the Endurance position had not changed and added that the proposals gave Aspen shareholders the chance to “express their frustration with the entrenchment of their board and management”.

“The refusal of the Aspen board and management to even discuss Endurance’s highly attractive offer serves as clear evidence that Aspen’s board and management only pay lip service to the interests of Aspen’s shareholders and are not interested in creating real value for them.”

Aspen’s chairman Glyn Jones has laid out the case for continuing as a stand-alone company and argued that Endurance’s offer undervalues the company.

Endurance has argued that the combination would create a larger company that could compete more effectively and reduce costs across a larger corporate base.

Endurance has set July 25, 2014 as the target date for voting on its two shareholder proposals.