Following her statement to the House of Commons on 9 October 2017 concerning the progress of Brexit negotiations, the Prime Minister was asked the following question by Ben Bradshaw MP:

Is it the Prime Minister’s understanding that, if necessary, it is possible to halt the article 50 process?

The Prime Minister replied:

The position was made clear in a case that went through the Supreme Court in relation to article 50. The Government have made it clear that we have no intention of revoking that. We will be delivering on the vote of the British people.

A reasonable inference from this response is that the Prime Minister was asserting that the Supreme Court, in the Miller case, had somehow ruled upon — by ‘making clear’ — the position regarding the revocability of Article 50. That interpretation of the Prime Minister’s response to Ben Bradshaw is reinforced by a subsequent exchange, this time between the Prime Minister and Chris Leslie MP. The latter asked:

May I press the Prime Minister to clarify her answer to my right hon. Friend the Member for Exeter (Mr Bradshaw)? He was not asking about Government policy: he was asking a straightforward question. Have the Government have received any legal advice that the article 50 notice can be revoked?

To which the Prime Minister responded:

I said to the right hon. Member for Exeter (Mr Bradshaw) that the position in relation to the revocation of article 50 was addressed by the Supreme Court in a case that went before it. It was very clear about that. We were clear as a Government that we were not revoking and it was clear in its consideration of the case of no revocation of article 50.

Again, the implication appears to be that the Supreme Court, having ‘addressed’ ‘the position in relation to the revocation of article 50’, had somehow ruled upon it. Only later did the Prime Minister’s position appear to shift somewhat, when, after being pressed by Pat McFadden MP, the Prime Minister said that: ‘The Supreme Court was clear that it operated on the basis that article 50 would not be revoked.’ It is unclear what the ‘it’ refers to here, but if it refers back to ‘the Supreme Court’, then perhaps the Prime Minister intended suggest that it — i.e. the Court — proceeded on that basis, which is factually accurate, the key point being that there is a difference between a court proceeding on the basis that something is true and deciding that it is so.

However, it was only in response to a very direct question that the Prime Minister’s position became more firmly and clearly rooted in what the Supreme Court did and did not decide in Miller. Helen Goodman MP said:

I am sure the Prime Minister knows that the Supreme Court did not opine on the question of whether article 50 was revocable, because the question before it was about our involvement. Therefore, why, when asked by my right hon. Friend the Member for Exeter (Mr Bradshaw) what advice she had had recently, did she rely on the Supreme Court?

The Prime Minister responded:

The point that I made in relation to the Supreme Court is that the court proceeded on the basis that article 50 would not be revoked; and I gave the answer to another of the hon. Lady’s hon. Friends about what the Government do or do not say about legal advice.

The Prime Minister was right to clarify her remarks in this way. The Supreme Court proceeded on the basis that Article 50 was irrevocable without deciding whether that assumption was an accurate one. The Court explained why its reasoning was framed in this way at paragraph 26 of judgment in Miller:

In these proceedings, it is common ground that notice under article 50(2) (which we shall call ‘Notice’) cannot be given in qualified or conditional terms and that, once given, it cannot be withdrawn. Especially as it is the Secretary of State’s case that, even if this common ground is mistaken, it would make no difference to the outcome of these proceedings, we are content to proceed on the basis that that is correct, without expressing any view of our own on either point. It follows from this that once the United Kingdom gives Notice, it will inevitably cease at a later date to be a member of the European Union and a party to the EU Treaties.

To be absolutely clear, then, the Supreme Court in Miller did not decide that Article 50 cannot be revoked. It simply decided the case on that assumption in the light of the way in which the parties had chosen to argue their cases.