It is sensible politics for Mr Corbyn to set as a condition of talks with Mrs May that she “takes no deal Brexit off the table.” That is because this is something that is beyond the power of her government. As she cannot do it, no talks take place, and Mr Corbyn ensures that he takes no responsibility for what is to come. (Save that he whipped his MPs to vote to trigger article 50 which set no deal as the default.)
How could no deal be prevented? There are only two options, their conditions are different under UK domestic law and EU law, and neither is under the control of the government.
The first is the approval of the 599 page Withdrawal Agreement. So far as the relations between the UK and EU27 are concerned, that is all that needs to be done. The 26 page Political Declaration, which sets out a possible framework for the future relationship, is a mere statement of intent. The future relationship is still all to be agreed. The Withdrawal Agreement is essentially non-negotiable. The Political Declaration is not only negotiable, nothing is settled at all. It is now a Golden Rule for those interested in Brexit that any article or interviewer that fails to draw the distinction between the two can be ignored.
Under UK domestic law however, both the Withdrawal Agreement and the Political Declaration must be approved under the European Union (Withdrawal Act 2018. As the Commons rejected them by 230 votes, this is not currently an option in Mrs May’s power.
The second is the revocation of the notice under Article 50. It is the government, not Parliament, that acts for the United Kingdom in international law. The Court of Justice of the European Union in Wightman decided that the UK does have the power to unilaterally revoke this notification. So, what would prevent Mrs May from doing so?
The government may not use its prerogative powers to ‘frustrate’ an Act of Parliament. Laws made by the legislature cannot be overturned by the executive. Section 1 of the European Union (Withdrawal) Act 2018 states that the European Communities Act 1972 is to be repealed on exit day. In order for that not to be so there would have to be no exit day. For the government so to act is flatly incompatible with the wording of the legislation.
Some might argue that the European Union (Withdrawal) Act expressed no intention to withdraw from the European Union, but merely provided a tidying up machinery for a decision taken by the government. This is both unpersuasive and irrelevant. The question is whether the government in purporting to exercise its powers would be doing so incompatibly with the terms of enacted legislation. It would.
This is a much more straightforward issue than that before the UK Supreme Court in Miller where the dissentients (rightly) argued that notification was not inconsistent with the terms of the European Communities Act.
Legislation would therefore be required to authorise the government to revoke article 50.
So, what legislation could make remain the default, rather than no deal Brexit? The easiest would be an amendment to the European Union (Withdrawal) Act, or a freestanding short Act, along the following lines
X. Duty to revoke notification of withdrawal from the EU
(1) If Y days before exit day no approval of the outcome of negotiations with the EU has occurred in conformity with section 13 of the European Union (Withdrawal) Act 2018, the Prime Minister shall notify the European Council of the United Kingdom’s revocation of its intention to withdraw from the European Union under Article 50(2) of the Treaty on the European Union.
(2) Upon such notification, the sections of the European Union (Withdrawal) Act 2018 specified in schedule Z shall be repealed
1 For the purpose of section X(2), the relevant sections are [all of them except 13].
A call by Mr Corbyn for legislation of this kind, which changed the default result, would be coherent. It is, however, very difficult to contemplate the current government trying to do so and not falling. Mrs May might call for such legislation, but this would remind me of the only joke in Shakespeare I ever found funny
I can call spirits from the vasty deep.
Why, so can I, or so can any man;
But will they come when you do call for them?
Addendum: For Lawyers Only
The frustration argument above is in fact slightly more complex. Section 1 is not yet in force. Section 25(4) provides that it is brought into force on such day as a Minister of the Crown appoints by Regulation. If the government could prevent there from being an exit day (by giving notification of withdrawal to the EU) section 1 could not be brought into force, by this or any future government. Section 25(4) would therefore be frustrated. because section 1 was. (A near identical issue was that which arose in the decision of the House of Lords in R v Secretary of State for the Home Office, ex p. Fire Brigades Union).
Another argument might be that the European Union (Notification of Withdrawal) Act 2017 implicitly gives the government authority to revoke the notice. Even if we accept this, which seems a stretch given the Act’s words and purpose, it is no longer arguable. The problem with revocation by the government alone is its inconsistency with the terms of the subsequent 2018 Act. Any such implied statutory power is therefore ruled out for the same reason as any prerogative power.