Operation Yellowhammer is the codename used by the UK Treasury for cross-government "no-deal" contingency planning for the possibility that Brexit negotiations for the withdrawal from the European Union fail to reach an agreement or an extension by 29 March 2019.[1]
Should there fail to be a negotiated settlement by that date, the UK would leave the European Union, leaving a hiatus impacting on many aspects of the relationship between the UK and European Union. The areas affected would include money, citizens, trade, customs and a range of regulations.[2]
On 6 September 2018 a press photographer captured a snapshot of a "no-deal" planning document revealing some details of then-current preparations and the HM Treasury codename for those plans.[3] The document appeared to indicate the Civil Contingencies Secretariat, a department of the Cabinet Office responsible for emergency planning, had been used in anticipation of government policy.[4] No further details were revealed.