Operation Yellowhammer - HMG reasonable worst case planning assumptions | The Guardian

Operation Yellowhammer is the codename used by the UK Treasury for cross-government civil contingency planning for the possibility of a no-deal Brexit. In the event of exit with no-deal, the UK's unilateral departure from the EU could disrupt, 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 is intended to mitigate, within the UK, the effects of this disruption, and would be expected to run for approximately three months. It has been developed by the Civil Contingencies Secretariat (CCS), a department of the Cabinet Office responsible for emergency planning. When the UK ceases to be a member of the EU in October 2019, all rights and reciprocal arrangements with the EU end.

The attached document is the leaked report referred to in the Guardian article.