The CME Group, the world's largest futures exchange, yesterday sweetened its bid for Nymex by $113m, focusing its efforts on winning over members of the New York energy exchange while keeping its offer to shareholders unchanged.

The CME's takeover has been bitterly criticised by some activists within Nymex, both by members who feel the Chicago exchange was offering too little and by shareholders upset at how the value of the bid was tied to the CME's beleaguered share price.