Leaving the EU
There is a mechanism set out in Article 50 of the Lisbon Treaty, whereby a member state can leave the EU. First, the state makes the decision to withdraw “in accordance with its own constitutional requirements”. It then notifies the European Council of its intention (known as “triggering Article 50”). After that, there is a two-year period for the departing state to negotiate the arrangements for withdrawal, including future relations between the state and the EU. Whether or not any such negotiations have been concluded, the departing state ceases to be an EU member at the end of that two-year period (unless all member states agree, unanimously, to extend the period).