This legal argument explains that, since the EU referendum was advisory and not in itself definitive, the Government has a duty to consider the decision properly, looking at all relevant factors, and not confine its attention to the simple majority outcome of the referendum. Any decision must consider the natonal interest and take into account the narrow margin of victory for Brexit and the large proportion of voters who did not participate, including citizens abroad who were deprived of a vote and should have been included.
A decision to leave the EU, taken after such proper consideration, must be made in an explicit and unambiguous Act of Parliament. The government does not have the power to simply go ahead with Brexit without Parliamentary approval. Also, based on earlier decision of the Supreme Court ["Miller"], Parliament must be explicit in its decision, it is not acceptable to be evasive, ambiguous or to just somehow imply its decisions by proceeding as though the decision has already been made.
Parliament has not, as a matter of fact, passed an Act deciding to leave the EU. Without such an Act, there is no basis for a S50 letter. So all in all, the government had no grounds for submitting its S50 letter to the president of the EU and that letter is legally meaningless. If the government continues to try negotiating withdrawal with the EU it will be open to challenge through judicial review, and can be ordered by the courts to stop.
just as it is conceivable that any member state might seek in the ECJ to challenge the validity of the UK government’s contention that the UK has decided to leave, it is conceivable that if the government persists in the charade of claiming that the constitutional decision was made by the Referendum there must assuredly be counterpart legal redress available domestically. Any concerned UK citizen with the necessary funds might seek on ultra vires grounds to ban the government from executing a treaty of withdrawal under Art 263 of the 1969 Vienna Convention whatever the result of the negotiating process. Indeed it would conceivably be feasible to seek to prohibit the start or continuation of the negotiating process designed to procure that end, and here we shall be back to Miller. The delivery of the 29 March letter was a legal non-event and so probably not in itself unlawful, but beginning negotiations with a view to a making a withdrawal treaty not sanctioned by the UK Parliament would be likely to be unlawful. This is because, as Miller made clear, the government enjoys no unilateral prerogative power to withdraw from the EU.