Operation Yellowhammer was the codename used by the British HM Treasury for cross-government civil contingency planning for the possibility of Brexit without a withdrawal agreement – a no-deal Brexit. Had the UK and EU failed to conclude such an agreement, the UK's unilateral departure from the EU could have disrupted, for an unknown duration, many aspects of the relationship between the UK and European Union, including financial transfers, movement of people, trade, customs and other regulations. Operation Yellowhammer was intended to mitigate, within the UK, some of the effects of this disruption, and was expected to run for approximately three months. It was developed by the Civil Contingencies Secretariat (CCS), a department of the Cabinet Office responsible for emergency planning.