Theresa May finds herself at the sharp end of negotiations with the EU to forge a final Brexit deal.

This week, the Prime Minister is attending a crunch European Council summit in Brussels where it was once hoped a sketch of that deal might be drawn up.

But expectations are low, after an unscheduled meeting between UK Brexit secretary Dominic Raab and the EU's chief negotiator Michel Barnier ended in an impasse.

The truth is, this divide has been there from the start and never went away.

Resolving the Irish border question is a binding requirement of any final withdrawal agreement - but it is by far the stickiest part of the negotiations.

First, the term that has been filling bulletins and headlines: this notion of a "backstop".

The expression comes from US baseball, and basically refers to a safety net or an insurance policy.

The problem, as yet intractable, is this: the UK wants to leave the European single market and customs union so it is no longer bound by European rules and so it can negotiate its own free trade deals.

However, the Republic of Ireland is a fully-fledged EU member, and were the UK to fully leave, it would require customs checks at the border with Northern Ireland.

Neither side wants this - it recalls the dark days of the Troubles, when such checkpoints were paramilitary targets, and would unravel the Good Friday peace agreement signed in 1998.

As part of her Chequers plan, Theresa May has proposed a backstop whereby the entire UK stays partly and temporarily within the customs union beyond the transition period that ends in December 2020.

It would stay in that customs arrangement until the border issue is worked out.

But for the European Union, and especially Ireland, there has not been enough time to work out the detail of this backstop ahead of signing the withdrawal deal - so they want another.

Their insurance policy would keep Northern Ireland within the EU's market and customs framework, even if the rest of the UK left.

However, the idea of a Northern Ireland-only backstop is wildly unpopular with Theresa May, who says she won't do it.

It's also opposed by the Conservative and Unionist Party and the Northern Irish Democratic Unionist Party (DUP).

They all say a separate customs framework for Belfast would threaten the integrity of the United Kingdom.

And Scottish Tory leader Ruth Davidson and Scottish secretary David Mundell reportedly wrote to the Prime Minister suggesting they could resign if Northern Ireland got a differentiated deal.

For completely different reasons, the SNP also opposes Northern Ireland getting a special deal - unless Scotland gets the same package too.

Nicola Sturgeon's party says Northern Ireland would be put at a huge competitive advantage among UK nations if it was the only one of the four still within the European economic framework.

On the other side, neither Irish nor EU leaders seem willing to budge on their two key principles.

Those are that the Irish border must be kept free of checks, and that the integrity of the European single market and customs union must be protected.

In other words, any part of the UK which wishes to stay in the trading bloc must abide by Europe's court and laws, with no exceptions.

Again for completely different reasons, the hard Brexiteers on the right of May's party also don't like the idea of a temporary customs arrangement for the whole UK while the Irish border is worked out.

Many of them want a looser trade deal with the EU in the style of Canada which would allow the UK to break with the single market, customs union and freedom of movement.

They fear a supposedly temporary customs arrangement without a well-defined time limit could drag on indefinitely.

No matter what, whether it's deal or no-deal, the Prime Minister will have to present her plan to Parliament.

Theresa May called a snap election last year hoping to strengthen her party's position in the House for crunch votes such this one, likely to occur by the end of this year or early in 2019.

Unfortunately for her, it didn't go to plan, and last June's election stripped the Tories of their majority, pushing them into minority government.

If she goes to the Commons to ask for a no-deal Brexit, it is unlikely MPs would approve such a course, and May could be forced from office for her failure to negotiate an agreement with the EU.

This could spark a Tory leadership election, or a general election, or both, with a fresh visit to the polls the preference of Jeremy Corbyn's Labour party.

If she offers Parliament a deal, the faultlines within her own party - on what kind of Brexit, on the Northern Irish question and more - will come to the fore.

As for Labour, it wants a deal that protects jobs through a permanent customs union, while the SNP has vowed to vote down any deal that does not retain the Scotland's single market membership.

In minority government, the Conservatives rely on Arlene Foster's DUP to prop them up and get their bills passed.

But with the Northern Irish unionist party's hardline opposition to any deal which dislocates it economically from the wider UK, the sticky issue of the backstop resurfaces again.

Finally, if enough MPs backed such a move, a second EU referendum on May's deal could be tabled - a so-called 'People's Vote' - as is the preference of Vince Cable's Liberal Democrats.

It is not only the UK Parliament which needs to ratify any agreement - 20 out of 27 EU members must agree at European Council level, with at least 65% of the European Union's population represented, before it is put before the European Parliament.

Meanwhile, in Scotland, Nicola Sturgeon continues to assert her government's right to take a second run at an independence referendum in the event of a bad Brexit deal or no deal.

The stakes are high this week for the Prime Minister and will only get higher for as long as a deal is not agreed.

Even if she gets a deal, getting it through Parliament is a daunting prospect.

Were MPs to reject May's agreement, the UK would enter uncharted territory just as it would in a no-deal scenario.

The prospect of the latter has resulted in extensive contingency planning at all levels of government, with preparations to safeguard supply routes for necessities like food and medicines well under way.

The UK Government has long insisted such a prospect is unlikely, and has played down fears of shortages and civil unrest.

As things stand, it is written in law that Britain will leave the European Union at 11pm on March 29, 2019, followed by a transition period to ease the UK out of the trading bloc of at least 21 months.

But that's only if there is a deal, and that deal can be pushed through Parliament.

If there's not, it's difficult to predict with any certainty what would happen next.