"It certainly looks like a low probability of success," Niederauer said in a Financial Times video interview in Davos, Switzerland. At the World Economic Forum, he and his Deutsche Börse counterpart, Reto Francioni, were hoping to take one final opportunity to convince EU Competition Commissioner Joaquín Almunia (left) to reconsider a decision to prohibit the $9 billion-plus deal.

With less than a week to go before the scheduled Feb. 1 vote by the full 27-member Commission, Niederauer conceded to having "misjudged" the regulatory process in Brussels, saying there was just a "glimmer of hope" the deal will go ahead.

The question then becomes whether the companies should stick to the original script and wait until the European Commission decision or call it quits now, as Deutsche Börse did in 1996 when it withdrew its unsolicited bid for Euronext -- now part of NYSE -- before the Commission ever had a chance to rule.

"At this late stage, whether [NYSE Euronext and Deutsche Börse] call it quits now or avoid the final verdict will make little difference," said Hirander Misra, managing director of London-based financial markets consulting firm Misra Ventures Ltd. "It isn't over until it's over, but the companies will probably see it through until the bitter end."

Once there is a merger prohibition on the books, experts agree, it would not be worth the time or expense for the companies to file an appeal.

"Once you get such a stringent pushback, it's almost then pointless to fight against the tide," Misra said. "They should just retrench, lick their wounds and then refocus on their core strategies going forward."

"Politically," said another expert, "an appeal would just amount to the managers pursuing a dead horse."

The deal would put more than 90% of the European exchange-traded derivatives market in the hands of one company. Europe's No. 1 and No. 2 derivatives exchanges, Deutsche Börse's Eurex and NYSE Euronext's London-based Liffe, respectively, would be under one roof. Exchange operators that the European market share are not significant because derivative trading is a global endeavor.

Deutsche Börse and NYSE Euronext have already tweaked concessions offered to European regulators to try to win approval for their deal, which they had been counting on closing soon after the EC decision. Deutsche Börse has also promised German regulators it would protect Frankfurt and nearby Eschborn as European derivatives trading hubs in return for getting the green light to merge.