What is the 'meaningful vote' and why is it important?

On Tuesday evening, between 19:00 and 21:00 GMT, UK MPs will vote on a deal reached by Prime Minister Theresa May with the European Union on the terms of Britain's exit from the bloc - "Brexit".

The widely anticipated vote has been dubbed "meaningful" because it reflects a concession made by May under intense pressure to allow parliament to ratify her plan.

It is the first time MPs have the chance to vote on the agreement after two years of difficult negotiations as the clock ticks towards the Brexit deadline of March 29.

The vote has been scheduled for December, but May postponed, expecting the deal to be rejected.

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Whatever the outcome, the vote will determine how - or whether - the UK proceeds with Brexit, and the parliament's role in that process.

However, at a deeper level, it reflects an effort by the British parliament to assert its role in one of the most important political issues facing the country in generations.

Alice Lilly, a senior researcher at the Institute for Government in London, said: "You can view everything we have seen in the last few days and the meaningful vote itself in terms of the tension that exists between parliament and government.

"That tension between legislature and executive is always going to exist - but Brexit has really brought that to the fore over the last couple of years."

The good news, said Oliver Patel, research associate at the University College London European Institute, is despite many criticisms of the political process in the United Kingdom, the meaningful vote indicates its constitution is working.

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"There have been so many things that have happened during the Brexit process which the executive hasn't wanted that parliament has managed to get - which is how it should be."

However, cautions Paul Taggart, professor of politics at Sussex University's European Institute, this still does not resolve the divisions among MPs about what to do next.

"If parliament is reasserting itself and asserting sovereignty over this process, that is pretty much constitutional in the British context - but the more substantive point is what does parliament then do with that sovereignty if it cannot come up with a solution to Brexit?"

What are MPs voting for exactly?

Pariamentarians will vote on both a "withdrawal agreement" and a "political declaration" on the framework for the UK's future relationship with the EU agreed on November 25.

However, there is widespread opposition to aspects of May's deal among MPs of all parties, reflecting deep divisions over Brexit in the country.

Many members of May's own Conservative Party are incensed by the "Northern Ireland backstop" - an arrangement to ensure an open border with the Republic of Ireland that they say could lock the province or the entire UK into the EU permanently.

Prior to the meaningful vote, MPs are also likely to vote on a string of as yet unknown amendments put forward by both supporters and opponents of Brexit that could influence the dynamic of voting and eventual outcome.

This has meant that only the most foolhardy are daring to predict what will happen.

How will the vote shape Brexit?

The meaningful vote will ultimately determine the outcome of Britain's painful Brexit saga.

It will lead either to an amended withdrawal deal, a "no deal" Brexit that doomsayers claim will be economically disastrous, or a second referendum on these options or even whether to remain in the EU after all.

If May were to lose, she will probably seek concessions from the EU, then put her deal to parliament a second time.

Parliament could also hold "indicative votes" gauging support for alternatives to her plan such as arrangements modelled on Norway's relationship with the EU.

The opposition Labour Party has been pressing for another general election and threatens to engineer a confidence vote in May's government if she is defeated - but it is unlikely to win.

While there is also support for a second referendum on Britain's exit from the EU, it is not clear that a majority of MPs back this option.

What's expected to happen?

Most observers expect May to lose the meaningful vote, and her ministers admit this is likely.

However, the prime minister has made strenuous efforts to win support for her "compromise" and has moved between appealing to MPs hostile to Brexit and those who stridently support it.

Taggart said: "May has consistently tried to frame these big decisions in a way that suits her. She framed it last time as vote for her deal or there will be a 'no deal' Brexit. That didn't seem to work.

"So this time she has been saying: it is my deal or no Brexit, trying in particular to mobilise Brexit supporting MPs."

Patel said if May is defeated she could return to parliament with a modified deal that will be sufficient to secure the support of Conservative Brexiteers.

"There are a lot of Conservative MPs who don't want to vote against this deal and it might actually take less to persuade them to vote for a variant of it in the future than we can imagine - especially if it were to become clear in an indicative vote that a majority of MPs support a second referendum."

This means a key theme of the meaningful vote has become less about whether May will lose - and more by what margin.

Is May in a strong or weak position this time around?

The meaningful vote was initially scheduled to be held on December 11 but was postponed by May when it became clear she faced certain defeat.

Over the last two years, however, she has been in a uniquely strong position to shape the terms of Brexit, for two main reasons.

First, the British political process gives unique power to prime ministers to control the way parliament does business and set the terms of debate.

Second, Brexit has been such a divisive issue in the UK that the alternatives to it have been difficult to explain and understand, meaning no consensus exists about what to do.

"Broadly speaking, there is not a majority for any alternative - but there are majorities blocking nearly every alternative, so May has been able to frame Brexit because of the inability of other people to come forward with solutions," Taggart said.

Nonetheless, he added, at the end of the day this has left her weakened because while she has set the agenda she has consistently "kicked the Brexit can down the road".

"May is the architect of the corner that she is currently sitting in."