Kraft on Monday left open the door to increasing its hostile £9.8bn offer for Cadbury, refusing to rule out a higher bid to win over shareholders of the British confectionery group.

Cadbury defiantly rejected the formal bid from Kraft launched on Monday, describing the US food group’s offer as “derisory” after it did not alter the terms of its approach two months ago. Roger Carr, Cadbury’s chairman, declared the formal offer “worse than the proposal the board has previously rejected”.