London(CNN) Theresa May has embarked on many crucial weeks in her two-and-a-half-year premiership, thanks to Brexit, but these coming days look set to be the most decisive yet.
"We are going to live through an extraordinary week, writing an extraordinary page of our history, and not one of us knows the words on that page, what we will be reading on that page, in seven days' time," British constitutional expert Peter Hennessy said Sunday.
The focus will be the vote in the House of Commons on Tuesday evening on the Prime Minister's Brexit deal -- nearly two months after the withdrawal agreement was struck with the European Union.
But May's actions in the hours after the vote will be just as pivotal -- and that is the source of the uncertainty alluded to by Lord Hennessy in a BBC radio interview.
The government is expected to lose Tuesday's vote even though several of May's Conservative MPs have announced in recent days that they have dropped their opposition to her deal.
Even so, the margin of defeat could be in triple figures, a setback not suffered by a UK Prime Minister since 1924.
Such a crushing defeat could, in theory, trigger a prime ministerial resignation. But May has repeatedly insisted she is the one to finish the Brexit job, acting, as she says, in the national interest.
However, she will have to act fast and strategically to avert what could become a full-blown constitutional crisis and could trigger an opposition vote of no-confidence in her government.
If, as expected, her Brexit deal is voted down, the Prime Minister is required within three days to produce a Plan B acceptable both to the Commons and to EU leaders, who must approve any changes to the 585-page agreement.
The Commons is split over what that alternative should be. Many Brexiteer MPs want no deal at all, because it would completely sever ties with the EU; some MPs from both the Conservative and Labour parties are pushing for a "Norway Plus" arrangement under which the UK leaves the political union of the EU but retains close economic ties.
There have been reports of an attempt by MPs to seize control of the Brexit process from the government, although talk of a "coup" was dismissed as overblown since amending government legislation is at the heart of parliament's work.
May will travel to Brussels towards the end of the week to discuss any changes to the deal, as well as push for firmer assurances from the EU over the issue of the Northern Ireland backstop.
May has survived difficult weeks in the past -- including a confidence vote in her leadership by her own MPs last month -- but the potential for disaster this coming week is largely of her own making.
Tuesday's meaningful vote was supposed to have taken place in December, but was delayed by the Prime Minister as part of her strategy to "run down the clock" -- forcing MPs to accept her deal or risk a potentially damaging no deal when the UK leaves the EU on March 29th.
Running down the clock has served only to anger MPs on all sides, who instead used votes in parliament last week to try to block a no deal Brexit and to force the government into producing a Plan B within three days of this week's vote -- giving everyone more time to negotiate an alternative deal.
Yet the tight timetable could anyway be irrelevant given reports that EU officials are preparing for the March 29th deadline to be extended until July -- the first session of the European Parliament after the European elections in May.
The Prime Minister has already relinquished some control over her Brexit plans, yet this week could see her losing it altogether.