Just because you are a member of the European Union (EU) doesn’t mean that you have to ‘remain’ a member forever. Article 50 of the Treaty of the European Union provides for any member to withdraw from the Union in accordance with its own constitutional requirements.
Notification: According to the ‘remain’ campaign the required notification could be evidenced by the outcome of the ‘in/out’ referendum which has been won by the ‘leave’ campaign. This does not seem appropriate, nor practical, as states do not generally notify each other of such weighty matters other than by formal communique. This tends to be the case even if the correspondence stipulates the date from which such notification is to take effect.
Arguably, in the case of BREXIT it could be the date of the results of the poll.
This seems unlikely as it would unnecessarily truncate the time framework for working through the ‘exit’ process. In any event, the EU may refuse to commence the withdrawal procedure without formal notification under Article 50 of the Treaty of the European Union (‘The Treaty’) without reference to some event prior to the date of notification. After all it takes at least two to negotiate.
Negotiation: Next the withdrawing member will enter into negotiations with the European Council, under power delegated to the Council by the European Union to undertake such talks. The objective of the talks is to agree on a transitional arrangement with the leaving member and any future arrangements with the EU. The time frame for this is two years during which time the status quo remains.
As such the member who is withdrawing would be expected to participate in other EU business as normal, but it would not participate in internal EU discussions, or decisions on its own withdrawal.
On the EU side, the agreement would be negotiated by the European Commission following a mandate from EU ministers and concluded by EU governments “acting by a qualified majority, after obtaining the consent of the European Parliament.”
Take It or Leave It: The withdrawing member has no standing in the negotiations and is not part of the eventual outcome of the terms of withdrawal. Since the withdrawing member plays no part in the negotiations around the agreement it is presented with a fait accompli at the end of the negotiations. Any trade deal must also be approved by the European Parliament and can only be approved by qualified majority vote. However, if the agreement is reached on a ‘mixed’ one it would have to be approved by all the remaining members.
Footnote: I suspect that in the case of BREXIT this will be where the ‘real’ negotiations will start.