Brexit is hanging on a knife-edge as Britain's Prime Minister Boris Johnson scrambles to rally support for his last-minute divorce deal.

The UK and the European Union (EU) agreed to a "fair and balanced" agreement on Thursday, but it now needs to be approved by both the British and European parliaments.

The president of the European Commission, Jean-Claude Juncker, has written to the president of the European Council, Donald Tusk, asking him to endorse the deal, reports News.com.au.

But Mr Johnson faces the much tougher task of getting the deal ratified in the UK without a majority in parliament and with several opponents plotting against him.

Advertisement

WHAT'S HAPPENING TODAY?

MPs will gather in the House of Commons from 9.30pm NZDT Saturday (9.30am London time) to debate and vote on Mr Johnson's deal.

It's the first time parliament has sat on a Saturday since 1982.

Britain is due to leave the European Union on October 31. Photo / AP
Britain is due to leave the European Union on October 31. Photo / AP

Mr Johnson will begin by making a statement about the deal, which will kick start the debate. Then several politicians are expected to propose amendments, with one even calling for Britain to cancel Brexit altogether.

The actual vote is not expected to take place until after lunchtime (local time) at least.

"We expect that the debate will end at around 2:30pm (2:30am NZDT), although the debate on the motions is not time-limited and, depending on the progress of business on the day, the House might not 'rise' until slightly later in the afternoon," the House of Commons said.

HOW MANY VOTES DOES BORIS NEED?

Mr Johnson needs 318 votes in total to get the deal approved.

But his Conservative Party holds just 288 seats in the 650-seat House of Commons, so he will have to rely on support from other parties and independents to get over the line.

Britain's PM Boris Johnson shakes hands with French President Emmanuel Macron, (right), during a European Union leaders summit in Brussels, Belgium. Photo / AP
Britain's PM Boris Johnson shakes hands with French President Emmanuel Macron, (right), during a European Union leaders summit in Brussels, Belgium. Photo / AP

The Northern Irish Democratic Unionist Party (DUP) — a key ally of Mr Johnson's — has already said it will oppose the deal and lobby a faction of about 28 hard line Brexit supporters in the Conservative Party to do the same.

Advertisement

"We will be encouraging (other MPs to vote against it) because we believe it does have an impact on the unity of the United Kingdom, will spark further nationalist sentiment in Scotland and will be detrimental to the economy of Northern Ireland," DUP spokesperson Sammy Wilson said.

That means Mr Johnson will need Brexit-supporting Labour Party rebels to support his deal, as well as the 21 Conservative politicians who were expelled from the party earlier this year for voting against the government.

Nicholas Soames, one of the 21, said he would vote for the deal and thought most of his expelled colleagues would do the same.

Labour leaders have told party members to oppose the deal. But around 20 of them have previously indicated a desire to back a deal to honour the June 2016 Brexit referendum result.

WHAT HAPPENS IF BORIS WINS?

If Mr Johnson's charm offensive manages to corral enough votes to pass the deal, Britain will be on course to leave the EU in an orderly fashion on October 31.

However, parliament would still need to pass legislation to implement the decision.

WHAT HAPPENS IF BORIS LOSES?

If MPs reject the deal — as they did three times with former PM Theresa May's deal — a law passed earlier this year will compel the prime minister to ask the EU for another three-month delay. That will push the Brexit deadline to January 31, 2020.

Mr Johnson has said he won't do that, but has also promised to obey the law — a contradiction he's yet to explain.

The message from his advisers is: "New deal or no deal but no delay".

"His alternative option is to secure the approval of MPs for leaving the EU without a deal," the House of Commons said.

It's crunch time for Britain's Prime Minister Boris Johnson as MPs prepare to vote on his
It's crunch time for Britain's Prime Minister Boris Johnson as MPs prepare to vote on his "great" new Brexit deal. Photo / AP

WHAT IF IT'S A TIE?

If the vote is a tie, then Speaker John Bercow will hold the deciding vote. Traditionally that means he'll seek to keep the issue open for further discussion.

WHAT ARE THE EXPERTS SAYING?

It's a complex situation and not even expert analysts can gauge the exact numbers.

"There seems to be an indication that it will be very, very close. It will come down to one, two or five people," Joelle Grogan, a senior lecturer in UK and EU law at Middlesex University, said.

UNDERSTANDING BREXIT: The terminology explained

ARTICLE 50:

Article 50 of the European Union's Lisbon Treaty sets out the procedure for a country wishing to leave the bloc and imposes a two-year countdown to that country's departure. Britain triggered the process on March 29, 2017, and was due to leave on March 29, 2019. Amid deadlock in Britain's Parliament, the EU agreed an extension until April 12 and then until October 31.

BACKSTOP: The Brexit backstop is an insurance policy designed to ensure there are no customs checks or other border infrastructure between the UK's Northern Ireland and EU member Ireland after Brexit. The backstop says if no other solution is found, Britain will remain in a customs union with the EU in order to keep the Irish border open. Opposition to the backstop from pro-Brexit British lawmakers is the main reason the deal has been defeated in Parliament.

BREXIT: A contraction of "British exit," Brexit is Britain's departure from the European Union. The UK joined the bloc in 1973, and held a 2016 referendum on its membership that was won by the "leave" side.

BREXITER/BREXITEER: A supporter of Britain's exit from the European Union.

CUSTOMS UNION: The European Union customs union makes the 28-nation bloc a single customs territory, with no tariffs or border checks on goods moving between member states. It also has common tariffs on goods entering the bloc from the outside.

EUROPEAN UNION: Formed in 1957 as the European Economic Community by France, West Germany, Italy, Belgium, the Netherlands and Luxembourg, the group is now a 28-nation bloc of more than 500 million people with substantial powers over member nations' laws, economies and social policies.

HARD BREXIT: A Brexit that sees the UK cut many of its ties with the EU, including leaving the EU's vast single market and customs union. Some supporters of the idea prefer the term "clean Brexit," and say it will enable Britain to forge its own trade deals around the world.

INDICATIVE VOTE: Britain's Parliament has held a series of non-binding "indicative votes" on various Brexit outcomes as a way of finding out whether any have majority support. Lawmakers rejected every option, from leaving the EU without a deal to holding a new referendum on whether to remain.

LEAVER: A Briton who voted to leave the European Union. See also Brexiteer.

NO-DEAL BREXIT: If Britain and the EU do not finalize a divorce deal, Britain will cease to be an EU member without an agreement setting out what happens next. A no-deal Brexit would rip up the rules that govern ties between the UK Many businesses say that would cause economic chaos.

PROROGATION: Suspending a session of Parliament without dissolving it. Johnson's move to do this will keep Parliament away for five weeks in the run-up to the Brexit deadline. He says this is necessary to set out his government's future policies, but opponents believe he is trying to reduce the amount of time available for debate and legislation that could block a no-deal Brexit.

REMAINER: A Briton who voted to stay in the European Union.

SOFT BREXIT: A Brexit that sees the UK retain its close economic ties with the EU, including membership in the bloc's single market and customs union.

WITHDRAWAL OF THE WHIP: A political party deciding that a lawmaker does not represent it in Parliament any more, effectively suspending that person from the party. Johnson is threatening to do this to any Conservative lawmakers who support the opposition's attempts to block a no-deal Brexit.

WITHDRAWAL AGREEMENT/POLITICAL DECLARATION: In November 2018, Britain and the EU struck a two-part divorce agreement. It consists of a legally binding, 585-page withdrawal agreement setting out the terms of the UK's departure, and a shorter, non-binding political declaration committing the two parties to close future ties. The agreement must be approved by the British and European parliaments to take effect, but Britain's Parliament has rejected it three times. Johnson has insisted it is unacceptable and is seeking to renegotiate it.