On Tuesday, MPs will face something rare: a Commons motion which really does deserve to be described as momentous. It will set Britain’s place in Europe and in the world for years to come. The vote will place an especially heavy burden on Conservative MPs, for they have the power to inflict a hefty defeat on their own government, an administration which has no majority and which governs thanks only to a confidence and supply agreement with the DUP. It is all too easy to see where defeat on Tuesday could lead: to the collapse of the government, a general election and the arrival of Jeremy Corbyn in Downing Street.

Theresa May’s deal has been rejected by MPs on the left and the right, by radicals and moderates. It promises to leave us in a Brexit purgatory, neither in nor out, obliged to accept EU regulations and rules on trade without having any say in the making of those rules. MPs might accept a temporary transition if a free-trade deal was guaranteed to follow. But the reason that her government was the first ever to be found in contempt of Parliament was its refusal to release legal advice that shows there are no guarantees, and no guaranteed exit from a backstop that is described as temporary.

So far, more than a hundred Conservative MPs have said they will vote against the deal. This number will almost certainly shrink by the time of the vote, but all opposition parties say they will oppose the deal — bookmakers are offering odds of four-to-one on the bill passing. Afterwards there will be huge pressure on her to resign, possibly as a price for the DUP agreeing not to bring down the government.

How the rebels behave following the expected defeat will be crucial to the future of the country. It is quite possible that her signature Brexit plan, into which she has vested what remains of her authority, suffers the largest defeat in parliamentary history. If so, she might resign. If a new leader is needed, the process will have to be very rapid — something which is hard, but not impossible, to achieve under the current rules covering Tory leadership elections. It would not be acceptable for the party to indulge in a two-month leadership election campaign while the clock ticks down to a no-deal Brexit on 29 March. The process would have to be condensed into a matter of days.

Then, the first priority of the Prime Minister — whether it is May or not —will be to go back to Brussels and explain the fundamental problem with the deal: the inability of the UK to get out of the backstop unilaterally. It must be emphasised that this aspect of the deal cannot be sold to the UK parliament. It should be emphasised that even if the UK cannot leave the backstop unilaterally, there must be a mechanism that would allow genuinely independent arbiters to decide if an exit is justified.

EU leaders, who have outshone the Brits with their negotiating discipline, state that the current deal is the only one possible. But would they really sit on their hands refusing to talk any further as the deadline for a ‘no-deal’ Brexit approaches? If they also dislike the backstop, as Michael Gove assures us they do, why not agree to change it?

They would have every incentive to re-open negotiations, knowing that failure to do so is guaranteed to produce an especially poor outcome for Ireland, in whose trading interests the whole concept of the backstop was devised. A no-deal Brexit would also lead to what the backstop is meant to avoid: a hard border on the island of Ireland. A tweak to the deal, say, allowing Britain to exercise a right to exit the backstop if a trade deal cannot be agreed by a certain date not too far into the future, might be all it takes to win the support of the Commons. Most MPs are pragmatists and would be prepared to accept a few years of taking orders from the EU if, at the end of it, we could either move to a free-trade deal or pursue other options.

Throughout the Brexit process, May’s government has come across as obstructive, inept and intransigent when dealing with Parliament while appearing fatally meek before EU negotiators. Being found in contempt of Parliament this week for refusing to publish its full legal advice on the withdrawal deal is yet one more badge of shame, just as was fighting so hard to try to deny Parliament a vote in the enacting of Article 50 — a vote the government won easily when it was eventually forced to hold it.

The government looks tired and without much in the way of achievements to its name. It has no vision. Yet for the moment the alternative would be far, far worse for Britain. The prospect of a Labour government is already driving investment from the country. Jeremy Corbyn and John McDonnell have threatened to seize 10 per cent of the stock of companies and to introduce wealth taxes. Their election would lead to a run on the pound and a steep fall in stock markets. Entrepreneurs would find new homes in countries where the government did not hold them in such contempt.

The Conservatives — if the word can apply to the warring tribes who face each other in the Commons next week — have made an almighty mess of the Brexit negotiations. If May’s premiership goes down with her deal, they will have a very brief moment to unite behind an alternative agenda. For their sake, and that of the country, it should not be missed.