It was supposed to be Brexit Decision Day – at least as far as the MPs are concerned – but that is now looking increasingly unlikely and it will probably leave us with more questions than answers.

The first question is what will count as a defeat for Mrs May?

Yes, she will probably lose the vote as all the opposition parties and many of her own backbenchers reject the package she brought back from Brussels.

If she loses by 100 or more, as some of the more optimistic Brexiteers have been touting, then yes that’s certainly a defeat – but given their failure to get the numbers right about how many MPs were demanding a vote on her leadership, not too great store should be placed on their predictions.

If, on the other hand, she only goes down by 30 or 40 votes, is that a defeat?

Certainly it is in terms of her package, but she could well feel sufficiently emboldened to go back to Brussels to ask them for further concessions, specifically on the so-called Irish backstop - the customs arrangement between Northern Ireland and the Republic that would come into force if we fail to negotiate a free trade deal with the EU by the end of 2020.

Her new suggestion that Westminster MPs should have the power to end the backstop without the agreement of Brussels, has meant that some of her backbenchers are now thinking about supporting the deal, not out of any great enthusiasm but as the only way of saving Brexit.

And that’s the key to Mrs May’s survival – fear by Tory MPs that if this (or a similar renegotiated) package goes down the alternative might not be ‘no deal’ and trading on what are called World Trade Organisation terms but what they fear the most – no Brexit, or Brexit in name only.

This is because the mood in Westminster appears to be slowly changing, and not in a good direction far as Brexiteers are concerned; and three options are emerging that worry them greatly.

First, there is talk of ‘Norway plus’.

That means, yes leaving the EU but instead joining EFTA – the European Free Trade Area (current membership Norway, Switzerland Iceland and tiny Liechtenstein).

The upside is that it would give us all the economic benefits of being members of the EU without having to be part of the

Common Agricultural or Fisheries Policies.

The downside, from the Brexiteers point of view, is that we would have to accept free movement of labour and would have no say in the rule-making process.

Second, it appears that contrary to what has been assumed, it now appears that we can just decide not to go ahead with Article 50.

We can withdraw from the process at any time before the end of March and resume our EU membership as if nothing has happened.

Sounds straightforward enough, but after the 2016 referendum result it is difficult to envisage the House of Commons, let alone the country, going along with such a move.

And third, there is the growing support for a second referendum.

Clearly, in 2016 the country voted to leave the EU.

But when we voted then how many of us had any idea of what we were voting for - both in terms of the complexities of undoing more than 40 years of integration and the high price we would have to pay for doing so?

After Tuesday (assuming the vote still goes ahead) Mrs May has decisions to make, but so too does Mr Corbyn.

If Mrs May loses does he immediately table a no confidence motion which, were he to win, could result in him moving into Number 10, either as a result of a general election or as PM of a cobbled-together minority government?

Most likely he will keep his powder dry in order to see Mrs May’s next move, which could well be going back to Brussels to say, “Hey guys, we need more concessions”; and when that fails (as it surely will) it’s back to Westminster to say “Hey guys I told you, that wouldn’t work”

If that happens – what then?

Mrs May can do one of three things.

First, she could call for a second vote (of MPs not people), second, she could call a vote of confidence in her Government (which the DUP say they will support, but gets us no further down the Brexit road) or third, she could call a General Election, but that’s highly unlikely with Tory and DUP MPs – Brexit or no Brexit - united on doing everything in their power to stop Jeremy Corbyn moving into Number 10.

In other words, your guess is as good as mine.

Ivor Gaber is professor of political journalism at the University of Sussex and is a former Westminster political correspondent