Despite today’s reports of growing Cabinet consensus on the need for interim arrangements in the immediate post-Brexit period, the Government remains split on the fundamentals. While Liam Fox talks about an implementation phase during which Britain will forge new global trading links, Philip Hammond refers to transitional terms and a future relationship in which Britain retains existing ‘patterns of business and trade’ with the EU.

The clash between these competing visions is becoming more entrenched within Whitehall. The Treasury is challenging the Department of International Trade directly to prove it can line up free-trade agreements with non-EU countries that can outweigh the loss of European trade associated with leaving the customs union. Whether the Government can hold itself together as it is forced to confront these choices is open to question.

But while the focus has been on the Government ever since the election, it is increasingly evident that Parliament will have the decisive say over Britain’s approach to Brexit.

That is because the passage of Brexit legislation will be critical in determining whether or not Britain is able to operate outside of the single market and customs union. The flagship Repeal Bill, which was presented to Parliament shortly before the recess, is intended to lift and shift 40 years of EU regulation into UK statute, alongside a host of other Brexit bills and regulations that are needed to transfer powers and clean up law in a number of areas.

That complex panoply of legislation is vital if Britain is to have a legislative and regulatory framework in place for when we exit the EU on 29 March 2019. If it is not, then the country will be plunged into chaos and uncertainty which will paralyse trade and movement of people, goods and services.

In short, what many people term ‘hard Brexit’ (the UK out of the single market and customs union), is only deliverable if the Government is able to pass through the Westminster Parliament an enormous volume of Brexit legislation within the next two years.

A softer Brexit, by contrast, in which the UK remains in the single market and the customs union, is deliverable in the absence of this legislative framework. Indeed if the various Brexit legislation is not passed in time then it is arguably the only sensible option – short of remaining inside the EU for an extended period – that would avoid chaos.

So, Parliament has enormous influence over the approach to Brexit that the UK will ultimately pursue. For all the focus on the votes around the Article 50 Bill in March, and the discussions about what final vote Parliament may be given on any deal, it is actually the passage – or non-passage – of domestic legislation in between that will prove crucial.

That reality will have a major influence over the parliamentary politics of the coming months. The Labour Party is itself deeply divided over Brexit. The hard left leadership is as committed to a hard Brexit as Theresa May, albeit for different reasons. Labour MPs in many midlands and northern seats which voted Leave oppose single market membership on account of arguments around free movement of people. But other Labour MPs, notably in London, vehemently back a ‘soft Brexit’, with some aiming to roll back the decision to leave altogether.

What can unite this group is action to bring down the Conservative Government and opposition to the Repeal Bill is likely to be considerable. Rebel Conservative MPs are unlikely to thwart the Bill at Second Reading. They may however join in with amendments that would seek to undermine the mechanics of the Bill or bind the hands of government on its negotiating position. The Repeal Bill is wide in scope and must be vulnerable to amendments on all aspects of Brexit, from continued membership of the single market and customs union through to issues about the payment of a financial settlement.

That will make for a difficult passage through the Commons and a treacherous one in the Lords, where the Government holds no majority even with allies. While peers will be wary about voting directly against the Repeal Bill they will undoubtedly push back on government moves to seize more delegated powers and may be more prepared to push back on associated but less prominent legislation.

The potential for defeat and delay is considerable, which will put the Government under serious strain. The decision to trigger Article 50 in March, before any of these steps had been progressed, may come to be seen as a major strategic error.