The vote the UK should leave the EU, while of momentous political significance, had no legal effect; the legislation providing for the referendum said nothing about the outcome.

The procedural requirements for withdrawal of a member state from the EU are set out in Article 50 of the Lisbon Treaty. Article 50 says once the withdrawal process is triggered, the member state concerned automatically leaves two years later without the benefit of any negotiated withdrawal arrangements unless an agreement on withdrawal arrangements has been concluded or the other 27 member states have unanimously agreed to an extension.

The trigger for this process is a notification to the European Council of a member state's intention, given by a member state which has "decided to withdraw from the Union in accordance with its own constitutional requirements".

Before the referendum, the Government and Parliament seems to have assumed that an Article 50 notification would be given by the Prime Minister utilising 'Prerogative powers' (executive powers held by the Crown at common law that are exercisable by ministers). The Prerogative is widely used in foreign affairs (including the making of treaties), which Parliament has largely left in the hands of the Government.

There are, however, well-established limits on the exercise of the Prerogative. One is that it cannot be used to undermine a statute; another is that it must not be used to frustrate the will of Parliament.

Based on a broad interpretation of these principles, it is argued by some constitutional lawyers it is not open to Government to turn a statute into a "dead letter" by exercise of Prerogative powers or to act in a way which cuts across the purpose of an existing statute.

The argument continues that because an Article 50 notification would have this effect concerning the European Communities Act 1972 (ECA) – whose purpose is to provide for the UK’s membership of the EU and for the EU Treaties to have effect in domestic law – Article 50 cannot be triggered without parliamentary approval.

It is also argued specific provisions of the ECA and the European Union Act 2011 require parliamentary authorisation for Article 50 to be triggered.

Other constitutional lawyers argue prerogative powers can be used by the prime minister to trigger Article 50, or Article 50 may be triggered with the authority of secondary legislation. A London law firm is bringing a case on behalf of a group of clients to ensure the Government will not trigger Article 50 without legislation.

It is established law that prerogative powers can be judicially reviewed. Accordingly, it is possible any decision by the government to trigger Article 50 using prerogative powers could be challenged in the courts.

The safest course for the government would therefore seem to be either to invite the courts to give a prior ruling on the issue or to put to parliament legislation that would authorise Article 50 to be triggered.

Of course, it is possible parliament may not be prepared to pass such legislation – either until the nature of the future UK/EU relationship became at least reasonably clear or at all (bearing in mind a majority of MPs were in favour of remaining and would take into account additional factors to the referendum vote in deciding whether, in the national interest, the legislation should be passed).

Article first published in Insurance Day