Key Brexit date What happens 31 January The UK leaves the European Union under the Withdrawal Agreement. A standstill “transition” phase begins. March The European Union is expected to agree its mandate for negotiating a new relationship after the transition phase. 1 July Deadline for extending the transition phase, if agreed between the UK and EU. Boris Johnson has said the UK will legislate to forbid such an agreement on their side. 31 December The transition phase under the Withdrawal Agreement ends, meaning the UK leaves without a deal unless one is in place.