Chicago ? The Chicago Board of Trade received the final go-ahead from shareholders Monday to join forces with its longtime rival in an $11.9 billion deal that will end its 159 years of independence but could make the combined all-Chicago company the world’s biggest exchange.

The offer by Chicago Mercantile Exchange Holdings Inc. to buy one of the nation’s oldest and largest financial institutions won approval at shareholder meetings for both companies after four months of back-and-forth bidding involving upstart IntercontinentalExchange Inc.

The two exchanges said the deal should close within days, creating a firm named CME Group, a Chicago Board of Trade company.

Pairing the Merc and Board of Trade would create the world’s largest one-stop futures and options market for everything from interest rates to pork bellies.