The purpose of the Withdrawal Agreement Bill is to put the Withdrawal Agreement into UK domestic law. Nothing that MPs add to it can change the text of what the UK and the EU agreed last week. For this reason it is wrong, whatever the merits of it, to try and add a customs union to this bill. Its function is not to determine the future relationship but to put the exit deal into UK law.
If MPs feel strongly that a customs union is the right choice for the UK – I don’t, but there is a significant group of parliamentarians who believe this – then they will have an opportunity to express that view very soon. Boris Johnson has accepted the so-called Nandy / Snell amendments which mean that parliament will get to vote on the negotiating mandate for the trade talks. If parliament backed a customs union in that vote, the government would be bound to try and negotiate one. Adding a customs union to this bill, though, risks non-ratification either because the EU says the UK has contradicted the treaty text or because it renders asunder the parliamentary coalition in favour of a deal.
From conversations I have had in the last couple of hours, I strongly suspect that the bulk of those elected as Tories who voted for a customs union in the indicative votes would not back a customs union amendment to the WAB because they understand that it would wreck the bill. If an amendment does pass, it will do so largely on the backs of those who want a second referendum. They will hope that the addition of a customs union will lead to the deal failing; reviving the prospects of a second referendum.
My sense is that if the government has the votes for a programme motion that would allow the UK to ratify this deal by the 31st, it will also have the numbers to knock back a customs union amendment.