The Chicago Board of Trade is facing growing pressure from shareholders to walk away from both competing offers for the second-largest US futures exchange and retain its independence or seek a richer bid next year.

The option of walking away has gained traction since the CBOT's annual meeting last week because the value of the agreed deal with the larger Chicago Mercantile Exchange has fallen to 31 per cent below the more recent unsolicited offer from the Intercontinental Exchange, now worth $10.6bn.