19 October 2019 sees an extraordinary sitting of Parliament to consider the revised withdrawal agreement and political declaration approved by the European Council on 17 October. While media attention has focused on the government's efforts to forge a majority in favour of the revised deal, a key amendment to the main motion might yet delay that vote, keeping open the possibility of a Brexit extension beyond 31 October unless the primary legislation required to deliver the deal is in place.

The government's motion for 19 October reads:

That, in light of the new deal agreed with the European Union, which enables the United Kingdom to respect the result of the referendum on its membership of the European Union and to leave the European Union on 31 October with a deal, and for the purposes of section 1(1)(a) of the European Union (Withdrawal) (No. 2) Act 2019 and section 13(1)(b) of the European Union (Withdrawal) Act 2018, this House approves the negotiated withdrawal agreement titled Agreement on the withdrawal of the United Kingdom of Great Britain and Northern Ireland from the European Union and the European Atomic Energy Community and the framework for the future relationship titled Political Declaration setting out the framework for the future relationship between the European Union and the United Kingdom that the United Kingdom has concluded with the European Union under Article 50(2) of the Treaty on European Union, as well as a Declaration by Her Majesty’s Government of the United Kingdom of Great Britain and Northern Ireland concerning the operation of the Democratic consent in Northern Ireland provision of the Protocol on Ireland/Northern Ireland, copies of these three documents which were laid before this House on Saturday 19 October.

If that motion were to pass without amendment, then the government would be free from the requirement set out in the Benn Act to request an extension to the Brexit deadline of 31 October.

An amendment tabled by Oliver Letwin MP, with cross-party support, would state that " this House has considered the matter but withholds approval unless and until implementing legislation is passed".

Explaining the purpose of the amendment, Oliver Letwin MP said:

The purpose of my amendment is, accordingly, to remove from the government’s motion the bits that would have had the legal effect of satisfying the Benn act conditions and would therefore have removed the need for the PM to seek an extension. It is important to note that, under the Benn act, the extension is maximally flexible, and automatically comes to an end just as soon as the UK completes an orderly withdrawal. So nothing in my amendment or in the Benn act itself in any way delays the actual departure of the UK from the EU immediately following the ratification of the withdrawal agreement. In short, my aim is to ensure that Boris’s deal succeeds, but that we have an insurance policy which prevents the UK from crashing out on 31 October by mistake if something goes wrong during the passage of the implementing legislation.

If passed, the Letwin amendment would, in effect, switch the order of events so that the government would have to bring forward the primary legislation required to implement the deal without first obtaining a House of Commons resolution confirming approval of that deal. The 2nd reading debate on the legislation would, in effect, be the moment at which Parliament's willingness to approve the revised deal is tested – perhaps allowing time for more effective scrutiny of the deal and to persist with demands for the government to produce economic impact assessments in relation to the revised deal.