The reasoning of the majority of the UK Supreme Court is undermined by Lord Reed’s rather beautiful explanation of the European Communities Act (-, ). But at least the majority decision is not based on the mere fallacy in the argument for the claimants – that is, the argument that because Parliament conferred rights, the government cannot take them away. It is a fallacy because Parliament conferred the rights in question by enacting that the treaties should have effect. The true rule is that the government cannot act contrary to what Parliament has enacted. The government does not act contrary to what Parliament has enacted if it cancels a treaty to which Parliament has given effect in UK law (see ).
The basis of the majority decision is, instead, this:
‘We cannot accept that a major change to UK constitutional arrangements can be achieved by a ministers alone [sic]; it must be effected in the only way that the UK constitution recognises, namely by Parliamentary legislation.’ 
That is it. The constitutional importance of this treaty is too great for a minister to cancel it. No authority is offered for the proposition that, if a treaty makes a major change to UK constitutional arrangements, it cannot be cancelled without an act of Parliament. The majority say that ‘This conclusion appears to us to follow from the ordinary application of basic concepts of constitutional law’ .
The treaty does have great constitutional importance (‘paramount importance’, as Lord Denning said in Blackburn’s case; see Miller ). Its constitutional importance does nothing to support the proposition that the law prevents the government from exercising its constitutional responsibility for and authority over treaties. That would require a decision to take the power away from the government. Parliament made no such decision, as Lord Reed shows. He () and Lord Carnwath () also show that it was constitutionally unnecessary for Parliament to take away the power, because of the accountability of the government to Parliament. Triggering Article 50, if it were done by prerogative, would be ‘subject to Parliamentary scrutiny in whatever way Parliament chooses’ (Lord Carnwath ).
So the ECA 1972 did not take away the government’s authority to trigger Article 50. But now the Supreme Court has done so.
Timothy Endicott is Professor of Legal Philosophy in the University of Oxford.
Click here for further analysis of the Supreme Court’s Miller judgment.