Aspen Insurance Holdings Ltd has adopted a so-called poison pill to discourage attempts from fellow Bermuda re/insurer Endurance to stage a hostile takeover of the company.

The plan, which will be in place for one year, will give shareholders rights allowing them to buy Aspen stock at discounted prices, if a person or group acquires ten percent or more of the company.

For passive, institutional investors the threshold is 15 percent. The move is designed to significantly increase the cost of a hostile takeover.

Endurance, which made public its attempts to acquire Aspen in a $47.50-per-share, cash-and-stock deal on Monday last week, immediately quickly condemned the move and stated its commitment to delivering on its offer.

Aspen shares dipped 2.1 percent to $44.45 on Thursday, compared with Endurance’s $47.50-a-share offer. Aspen had rallied 11 percent to $43.77 the day the offer became public.

Endurance shares also fell 0.9 percent to $50.93.

“The rights plan is designed to deter abusive tactics from being used in a proposed takeover, to ensure that shareholders receive fair and equal treatment in any proposed takeover of the company and to provide that any transaction would appropriately reward our shareholders and be beneficial to our company,” Aspen said in its statement this morning.

The rights plan is set to expire on April 16, 2015, but the Aspen board may terminate it before then, if the directors believe it’s in the best interests of shareholders.

Michael McGuire, chief financial officer of Endurance, criticised Aspen’s move in an Endurance statement on Thursday.

“At a time when the Aspen board should be seriously considering an opportunity to deliver significant value to its shareholders, it is instead focused on blocking them from receiving that value and on taking actions to entrench themselves,” Mr McGuire said.

“This is not a surprise given the lack of alignment and clear disdain Aspen’s board has shown for its shareholders in summarily rejecting our proposal without any discussion whatsoever.

“A poison pill is a well documented defensive step typically taken by an entrenched board of directors. It’s interesting Aspen’s board adopted a poison pill that divides their shareholders into different categories — good and bad, passive and active — a division that is currently the subject of litigation in an unrelated situation.”

Mr McGuire added that Endurance remained “fully committed to delivering our highly attractive premium offer to Aspen shareholders”.

Mr Charman said last week that he was prepared to buy $25 million of Endurance shares with his own money in connection with the deal which will add to the $30 million stake he purchased when he arrived at the company in May last year.

The offer amounts to a 40 percent cash and 60 percent Endurance common shares deal. Outlining the proposal, Endurance stated that each Aspen shareholder would receive either $47.50 per Aspen share, or all Endurance common shares (0.8826 Endurance shares for each Aspen share), or a combination of cash and Endurance common shares. The potential combination would create a company with annual gross premiums written of more than $5 billion and shareholders’ equity of $5 billion.

Endurance has said it wants to maintain the headquarters of the proposed combined company in Bermuda, with a significant presence in London, New York and other key markets and added that it expected the combined company to “generate synergies exceeding $100 million annually”, which would result in significant improvements in return on equity earnings per share in 2015.

In a letter delivered to the Aspen board, Mr Charman makes it clear that he values Aspen’s key staff. “The retention of key members of the Aspen management team, underwriters and employees will be critical to the success of the combined business,” the letter states

On Monday, Aspen chairman Glyn Jones described Endurance’s proposal as “ill conceived” and a “strategic mismatch” and he argued that it undervalued Aspen.

Endurance chairman and CEO John Charman said he had been asking the Aspen board for discussions since January this year, but they “refused to engage with us”.

Goldman Sachs is acting as financial adviser to Aspen and Wachtell, Lipton, Rosen & Katz and Willkie Farr & Gallagher LLP are acting as its legal advisers.