It's like the English Civil War all over again, though this time the Roundheads (Remainers) are fighting the Cavaliers (Brexiteers) through the courts rather than on the battlefields of the British countryside.

The background

On 1 January 1973 the United Kingdom joined what were then known as the European Communities. It was a condition of membership that Community law should be given effect in domestic law. Primary Legislation was required to achieve that. Consequently the European Communities Act 1972 was passed.

The European Communities have now become the European Union. Pursuant to the European Union Referendum Act 2015 a referendum was held on 23 June 2016 on the question whether the United Kingdom should leave or remain in the European Union. The answer given was that the UK should leave.

The battleground

The process for withdrawal is governed by Article 50 of the Treaty on European Union.

The Government (the Roundhead Brexiteers) believe that, as a matter of UK constitutional law, it is entitled to give notice of a decision to leave the European Union under Article 50 by exercise of the Crown's prerogative powers and without reference to Parliament. The Cavalier Remainers say that Parliamentary approval is required. A number of test cases were brought in the High Court to challenge the Government's position. On Thursday it gave its decision that Parliamentary approval was required. The Government has appealed to the Supreme Court. We understand that for the first time there will be a full sitting of all 11 justices of the Supreme Court. This will therefore be the most significant constitutional decision of modern times.

The constitution

The most fundamental rule of the UK's constitution is that Parliament is sovereign and can make and unmake any law it chooses. As an aspect of the sovereignty of Parliament it has been established (during the original English Civil War) that the Government of the day cannot by exercise of prerogative powers override legislation enacted by Parliament. This principle is of critical importance and sets the context for the general rule on which the Government seeks to rely - that normally the conduct of international relations and the making and unmaking of treaties are taken to be matters falling within the scope of the Crown's prerogative powers.

In the present case, however, the Government accepts, and indeed positively contends, that if notice is given under Article 50 it will inevitably have the effect of changing domestic law. Those elements of EU law which Parliament has made part of domestic law by enactment of the 1972 Act will, as a result of the operation of Article 50, in due course cease to have effect.

The central contention of the Government in the present case is that Parliament must be taken to have intended when it enacted the 1972 Act that the Crown would retain its prerogative power to effect a withdrawal from the Community Treaties (now the EU Treaties), and thereby intended that the Crown should have the power to choose whether EU law should continue to have effect in the domestic law of the UK or not.

The appeal

The High Court did not accept the argument put forward by the Government. It said that the argument was contrary both to the language used by Parliament in the 1972 Act and to the fundamental constitutional principles of the sovereignty of Parliament and the absence of any entitlement on the part of the Crown to change domestic law by the exercise of its prerogative powers.

We await the decision of the Supreme Court listed for hearing on 7 December.

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