Nasdaq OMX Group and IntercontinentalExchange Inc will take their $11.1 billion takeover bid for NYSE Euronext straight to the exchange operator's shareholders with a tender offer.

The move comes after NYSE's board twice rejected the pair's offer in favor of a $10.3 billion friendly bid by Germany's Deutsche Boerse AG, and it sets the stage for a lengthy battle for the Big Board's future.

The direct appeal to shareholders on Monday intensifies the pressure on NYSE, whose investors have urged it to at least talk to Nasdaq and ICE about the alternative deal.

The commencement of this exchange offer should convince the NYSE Euronext board of the seriousness of our intentions, Nasdaq Chief Executive Robert Greifeld said in a statement.

Nasdaq and ICE want to split NYSE Euronext's operations between them, and have said theirs is a superior deal for shareholders, who will ultimately have to decide.

NYSE's board has argued the unsolicited bid is not serious, it risks being blocked by antitrust regulators, and runs contrary to the company strategy of diversifying internationally. The Deutsche Boerse tie-up would create the world's largest exchange operator.

I am bothered that NYSE has avoided any conversation with Nasdaq. I feel as if they are biased to the Deutsche offer even though economically it's not equal, said Keith Wirtz, chief investment officer at Fifth Third Asset Management, which holds more than 100,000 shares of NYSE Euronext.

I am concerned that it has been driven by other factors, well beyond economics, that as a shareholder of NYSE don't make sense.

Nasdaq, which runs stock and options markets in the United States and Nordic European countries, and ICE, an energy futures specialist based in Atlanta, said their offer would be detailed in a regulatory prospectus to be filed this month.

The pair listed seven conditions that could yet complicate the tender offer.

In defense, NYSE could institute a poison pill or take out a chunk of debt to pay a special dividend. Such moves could thwart the hostile bid by making it more difficult and expensive for the rivals to buy its shares.

NYSE Euronext had no immediate comment.

I believe this will definitely pressure the NYSE board of directors to open discussions with Nasdaq OMX and the IntercontinentalExchange, said Michael Wong, a Morningstar analyst in Chicago.

NYSE Euronext shares were up 1.1 percent at $40.48, Nasdaq slipped 0.2 percent to $27.07, and ICE fell 2.2 percent to $117.74. Shares of Deutsche Boerse -- the largest of the four -- closed up 0.4 percent at 56.32 euros on Monday.

(Reporting by Jonathan Spicer and Paritosh Bansal; additional reporting by Ann Saphir in Chicago; Editing by Bernard Orr, Richard Chang, Dave Zimmerman)