The reverse termination fee, which would be paid by the bidding party in event of a failure of the deal, came today in the joint proposed merger agreement released by the two hostile bidders and is intended to alleviate fears of the deal being thrown out by US antitrust authorities.

In a highly charged statement, Nasdaq and ICE said the deal would “strengthen investor confidence in US equity markets” and, in a clear challenge to the Deutsche Börse bid, added it would “preserve competition in the European Union”.

Jeffrey Sprecher, chairman and chief executive officer of ICE, said, "Based on the feedback we have heard from NYSE Euronextstockholders, we are more confident than ever that the proposed NASDAQ OMX/ICE transaction is better for them, the markets and the exchange's customers.

“We trust that the NYSE Euronext Board will seek to enhance the value to its stockholders by meeting with us to evaluate our superior proposal."

In addition, Nasdaq and ICE said that they had $3.8bn financing in place for the bid from a syndicate of banks including Bank of America, Nordea Bank, Skandinaviska Enskilda Banken and UBS Investment Bank for the Nasdaq part of the bid and Wells Fargo and Bank of America for ICE.

The board of NYSE Euronext unanimously rejected the Nasdaq/ICE bid on 10 April citing concerns over the chance of completion and reiterating its belief that the group should not be broken up.

In a letter sent to Jan-Michiel Hessels, chairman of the board of directors at NYSE Euronext, and included in today’s statement, Nasdaq and ICE said: “We believe your shareholders should be given the opportunity to make the decision as to how best to allocate their interests in NYSE Euronext.”

As the Deutsche Börse deal is structured as an all share merger, there is no requirement for the NYSE Euronext board to seek the highest bid for the business. However, Nasdaq and ICE are hoping that the premium on their bid will be enough to provoke a shareholder rebellion over the acceptance of the Deutsche Börse bid.

The letter said: “The agreement with Deutsche Börse permits you to engage in discussions with us and allow us to undertake our due diligence investigation of NYSE Euronext if you believe that "there is a reasonable likelihood that our proposal could constitute a Superior Proposal." In this regard, you need not actually determine that our proposal is, at this time, a Superior Proposal.

“There is no question that our proposal is economically "Superior", indeed far superior, and we believe your duty to be well informed requires that you discuss with us any issues that could conceivably overcome this sharp difference in the value of the two offers.

“By engaging with us, you will be enhancing your ability, and the ability of your stockholders, to consider our proposal, and you will also discover whether Deutsche Boerse would improve its offer in response.”

NYSE Euronext will hold its annual shareholder meeting on the 28 April.