In a statement the FIA argues that CME and CBOT are already the two largest futures exchanges in the US, representing over 85% of the market in US futures exchange trading and the merger would "concentrate significant market power in the new CME Group".

The combination would "substantially lessen competition among US futures exchanges, and raise even higher the barriers to entry for new competitors," says the FIA.

But the lobby group does acknowledge that the merger could have short-term cost savings and operational efficiencies and stops short of calling for the deal to be scrapped.

The FIA says various market structure alternatives could potentially overcome the anti-competitive effects of the merger and it is considering internally the efficacy of those over-the-counter alternatives.

The group has previously expressed concern about the lack of meaningful competition among US futures exchanges and their clearing facilities. In 2003 the FIA came out in support of Eurex's ill-fated plans to set up a US futures exchange, despite continued opposition from the CME and Cbot.

The FIA's reservations about the Cbot-CME merger come as the two Chicago exchanges await the results of an antitrust review by the US Department of Justice. The merger must also be approved by the Commodity Futures Trading Commission and the SEC.

Despite this the two exchanges said last month that they were on track to complete the merger transaction by mid-2007. The parties have already filed a registration statement on form S-4 with the Securities and Exchange Commission and decided on a high level organisational structure and senior leadership team for the combined company.