The pressure on the Chicago Mercantile Exchange to sweeten its agreed bid for the Chicago Board of Trade intensified on Monday as the premium in the rival offer from the Intercontinental Exchange touched $1.5bn for the first time.

The Board of Trade last week said it was nearing the end of its review of the ICE proposal, with some directors continuing to push the CME to at least match the terms proposed by the smaller, Atlanta-based ICE.