Section 13(4) of the Act provides that, if the House of Commons 'decides not to pass the resolution' approving the Withdrawal Agreement, as it did on 15 January, a minister must make a statement setting out how the government proposes to proceed in relation to the EU exit negotiations 'within the period of 21 days beginning with the day on which the House of Commons decides not to pass the resolution'.

Section 13(6) of the Act requires that a minister must then make arrangements for 'a motion in neutral terms' to be considered by MPs within seven sitting days beginning with the day on which the Section 13(4) statement is made.

However, the Grieve amendment to the business motion of 9 January required the government to table a section 13 motion within three sitting days of the Withdrawal Agreement being rejected. The Prime Minister indicated after losing the 'meaningful vote' that the government would make a statement simultaneously with tabling the motion, and the Leader of the House, Andrea Leadsom MP, announced at Business Questions on 17 January that - subject to the agreement of the House - the debate on the motion would take place next Tuesday, 29 January. Assuming that the government identifies the process starting today as the section 13(4)-13(6) process, a process which under the EU(W)A could have taken 21 calendar + seven sitting days has thus been truncated to three sitting + six sitting days.

The EU(W)A requires the government to make the necessary statement in writing in such form as the Minister 'considers appropriate'. In theory, if anything substantial has emerged from the cross-party talks, today's written statement could be in the form of a White Paper laid before the House, rather than a normal written statement. The form that the written statement will take may not be clear until an oral ministerial statement, presumably by the Prime Minister, is made in the House this afternoon after Home Office questions.

One thing to look out for is whether the government indicates that the provisions of sections 13(7) - 13(12) of the EU(W)A also pertain to today's statement and the 29 January debate. These sections concern the statutory obligations on the government if, by the end of 21 January, there is 'no agreement in principle' in the Article 50 negotiations with the EU. The government's position - as set out, as required by the EU(W)A, in a statement laid before Parliament on 26 November - is that agreement in principle has been reached, as set out in the Withdrawal Agreement; it is the House of Commons, not the government, that has declined to consent to ratification of that Agreement. However, the Prime Minister may conclude that it would be wise to clarify the matter. Section 13(13) of the EU(W)A allows statements and motions under the section 13(4)-13(6) 'Withdrawal Agreement rejection' provisions and the section 13(7)-13(12) 'no agreement in principle' provisions to be combined into one. By rolling up the statements and motions in this way, ministers would ensure that their obligations under the section 13 provisions of the Act have been clearly discharged.

All section 13 motions apart from the 'meaningful vote' approval motion must be in neutral terms.

The section 13(6) neutral motion that the government will likely lay today will probably begin with the words, 'That this House takes note of ...' or 'That this House has considered...', and the words that follow will reflect the essence of the Prime Minister's statement. It is this neutral motion that will be debated on 29 January.

Neutral motions are not normally amendable. However, the Grieve amendment to the Business of the House motion on 4 December secured the House's support for all section 13 motions to be amendable. MPs can table amendments until the rise of the House the night before the debate, so once the government tables its motion today they will have a week to do so until the rise of the House on Monday 28 January.

In Business Questions on 17 January, the Leader of the House, Andrea Leadsom MP, indicated that the whole day would be set aside on 29 January to debate the government's motion. However, this is subject to the decision of the House on a business motion that the government will move on 29 January before the main debate.

The default procedural position would be that debate on the main motion would continue to 7pm; the Speaker would select one amendment, and, at 7pm, the matter could be talked out unless a Member moved a closure motion. Leadsom indicated that a Business of the House motion to govern the debate would be proposed instead, and that this would be both debatable and amendable. The provisions of this motion may not be known until late on 28 January. (For comparison, the Business of the House motion governing the 'meaningful vote' debate re-starting on 9 January was not published until the afternoon of 8 January.)

In its business motion for 29 January, the government could propose simply that the question should be put at 7pm and any amendments the Speaker may have selected can then be moved. The government may seek to limit the number of amendments selected (as it did in the first 'meaningful vote' Business of the House motion in December), or it may enable the Speaker to select as many amendments as he wishes (as it did in the second 'meaningful vote' Business of the House motion on 9 January).

Alternatively, and depending on the government's appetite for seizing the initiative, the government could lay a more elaborate Business of the House motion, permitting, for example, a different approach to decision-making (such as a series of indicative votes on a range of options) at the end of the debate or to be carried over for decision on a subsequent day. The government's business managers know that some backbenchers are preparing their own amendments for a range of possible decision-making models; if they do not pre-empt such efforts, they may be overtaken by them.

It is not possible to answer this question definitively until amendments are formally tabled and published. However, substantive amendments seeking to forge a new approach to the Brexit process – such as those mooted by Nick Boles/Yvette Cooper and Dominic Grieve – could potentially be laid to either the main motion or the Business of the House motion.

Amendments which seek to amend or rearrange Standing Orders, or to give priority to non-government business on other days, are arguably not in scope of a business motion which is expressly provided to organise the debate scheduled for 29 January only. In normal circumstances, amendments to the business motion would concern the number of hours of debate to be set aside, or the number of amendments that might be chosen. However, the Speaker's ruling in connection with the Business of the House motion on 4 December, which permitted the amendment from Dominic Grieve allowing amendment of a neutral motion scheduled for debate on another day, has set a recent precedent. Strong statements from the chair in recent weeks suggest that a restrained approach is not likely to be adopted.

In theory, as the neutral motion on the government's statement requires only that MPs consider the statement, any amendment which goes beyond this and suggests an alternative approach would also be beyond the scope of the main motion. However, in practice, this will not satisfy Members, and the implication of the Speaker's recent rulings and statements must be that if, in his judgement, the House wishes to reach a decision on something substantive it must be given a chance to do so. On what appears to be the Speaker's view, if there is any possibility of cross-party agreement on what to do, this must be given the opportunity to show itself; rigid adherence to procedure would not be a determining factor.

Moreover, after 29 January, once the section 13(6) motion in neutral terms is moved by the government (and assuming that the 'no agreement in principle' provisions have not been engaged), ministers' statutory obligations under the EU(W)A will have been discharged.

Fox, R. (2019), 'Plan B for Brexit': what will happen in the week ahead?, (Hansard Society: London)