The

InterContinental Exchange

(ICE) - Get Report

isn't melting under the pressure of a big bidding war.

The Atlanta-based futures and commodities exchange once again urged

Chicago Board of Trade

(BOT)

shareholders to reject a planned merger with the

Chicago Mercantile Exchange

(CME) - Get Report

. The ICE, which has made an unsolicited rival bid for the CBOT, comes two weeks ahead of a crucial CBOT shareholder vote.

ICE's current $11.9 billion bid is worth some $25 a share above the CME's offer, the letter says. ICE started the bidding war in March.

"Your board of directors has agreed to a bargain basement sale of your company to the Chicago Mercantile Exchange in a transaction that would leave over $1 billion of your money on the table," ICE said in the letter. "ICE's merger proposal is clearly superior, both financially and strategically, yet your board has failed to act in your best interests and continues to recommend the inferior CME transaction."

The CME -- which originally agreed to purchase the CBOT back in October -- has been forced to tweak its offer several times, giving CBOT shareholders a bigger stake in the combined company and setting a huge buyback once the deal is done.In addition, last week the CME added a one-time cash dividend to the mix.

But the ICE has also been upping the ante. Among other things, it's offering a portion of the deal in cash as opposed to all stock and has agreed with the Chicago Board Options Exchange to settle a long-running dispute over exercise rights held by CBOT members.

"This is well worth the fight, and whoever wins this wins a valuable commodity," says Michael Greenberger, a professor at the University of Maryland Law School and a former director for trading and markets for the Commodities Futures Trading Commission. "The futures industry is the hottest, most dynamic industry in the financial sector. It's overwhelming the equity and debt markets and the exchanges only have a small part of it because the OTC market is even more vast."

Still, no matter how much the ICE pushes for the CBOT, Greenberger says it's likely that the CME will win the battle.

"If you had to bet, the CME will prevail in this largely because what it's offering is based on a more sound and fundamental and historically stable basis," he says. "The ICE has obviously been incredibly adroit in the ways it's been managing its business, but it is the new kid on the block and doesn't historically have the stable foundation that the CME has."

Shares of the CBOT fell $1.19 to $208.28 on Thursday. The CME fell 29 cents to $543.61, while shares of ICE dropped 18 cents to $157.92.