First of all, it is important to note that the case was not about whether the Government should respect the will of the people or whether the result of the referendum gave the Government the power to leave the EU without reference to Parliament. It was accepted by the Government that there was no statutory power to do so.
Instead, the case hinged on the Royal Prerogative. This is an ancient power of the Crown which, according to the UK’s unwritten constitution, is invoked to make treaties. The issue the court addressed was whether this power could be used to exit not just the treaties governing the UK’s membership of the EU but also to remove UK citizens’ rights which were incorporated into British law by the European Communities Act 1972 (the ECA 1972).
The key points of the Article 50 High Court judgment
The Judgment, in reality, does not break new ground. It was common ground between the parties that Parliament (not the Crown) was supreme, by which it is meant that Parliament has “the right to make or unmake any law whatever; and further, that no person or body is recognised by the law … as having a right to override or set aside the legislation of Parliament”.
More than this, long-standing legal principles dictate that the prerogative cannot be used to override statute, or, for that matter, the common law. It was in the interaction between this idea and the idea that treaties are to be made and broken by the Crown that the decision fell to be made. The court summarised the Secretary of State’s principle submission as being that Parliament would have needed to have used clear words in the ECA 1972 to have ousted the Crown’s residual power.
The court found that this argument glossed over the clear constitutional background to the ECA 1972, stating that it “would be surprising indeed if… Parliament, as the sovereign body under our constitution intended to leave the continued existence of [EU rights] …subject to the choice of the Crown..” [para 86]
Further, “parliament having taken the major step of switching on the direct effect of EU law… by passing the ECA 1972 … it was not plausible to suppose that it intended that the Crown should be able by its own unilateral action under its prerogative powers to switch it off again.” 
Finally, the court referred to the status of the ECA 1972 as a constitutional statute, exempt from the ordinary doctrine of implied repeal (by a later inconsistent statute). In such circumstances, it could not be thought likely that Parliament “intended that its legal effects could be removed… through the use of … prerogative powers” .
As a matter of statutory interpretation, the court therefore held that it is was clear that “Parliament intended to legislate by [the ECA 1972] so as to introduce EU law into domestic law… in such a way that this could not be undone by exercise of Crown prerogative power… The Crown therefore has no prerogative power to effect a withdrawal… by giving notice under Article 50” .
Article 50 High Court ruling: what will happen next?
The Government will now appeal to the Supreme Court although such an appeal is not without risk.
Setting aside what I regard as the clearly correct decision on the interpretation of the ECA 1972, the Supreme Court may feel that it is necessary to decide a matter on which both parties were otherwise agreed: whether an Article 50 notification is irrevocable.
On an appeal, we may have the unedifying spectacle of a Government apparently committed to leaving the EU asking the European Court of Justice to determine a fundamental question relating to that process.
Such arguments may take years, extending the Brexit limbo and increasing the damage to the UK arising out of the uncertainty.
I hope that the Government now takes what was always the principled stance and introduces to Parliament a bill which will satisfy the country and enable MPs to act as they were elected to do. “Your representative owes you, not his industry only, but his judgment; and he betrays, instead of serving you, if he sacrifices it to your opinion” [Edmund Burke, Speech to the Electors of Bristol].
The EU Referendum was advisory. It is now time for Members of Parliament to consider that advice and exercise their judgment in service of the country.