EXPLAINER-'Super Saturday' Brexit showdown in parliament looms for PM Johnson

Prime Minister Boris Johnson faces a Brexit showdown with parliament on Saturday after clinching a last-minute divorce deal with the European Union that his Northern Irish allies oppose.

In an extraordinary Saturday sitting, the first since 1982, parliament will vote on approving Johnson's deal. Britain is due to leave the EU on Oct. 31. But Johnson, whose Conservatives have no majority in the 650-seat House of Commons, will face a deeply divided parliament where his opponents are trying to force both a delay to Brexit and another referendum.

Other options include collapsing his government so that others can take control of Brexit negotiations. WHEN IS PARLIAMENT SITTING?

The government has proposed that parliament sits from 0830 GMT on Saturday, Oct. 19 - the first time since April 3, 1982, when it discussed the Argentine invasion of the Falkland Islands. Lawmakers are expected to debate until 1330 GMT, although no end time has yet been set and votes could extend the session.

A spokeswoman for Johnson said lawmakers would vote on the deal. The legislation would still then need to be passed by Oct. 31 in order to ratify the agreement. Parliament still has to vote on Thursday on whether or not to go ahead with the Saturday sitting.

WHAT IS BEING DISCUSSED? Brexit.

Johnson said he had agreed a "great" new Brexit deal. If approved, Johnson can proceed with his plan to leave the EU on Oct. 31. If rejected, he may seek approval to leave the EU without a deal on Oct. 31.

The Democratic Unionist Party said it could not support the deal. The opposition Labour Party, the Scottish National Party and the Liberal Democrats have all said they will oppose it. If he loses votes on both a deal and no deal, he is required by law to write a letter to the EU requesting more negotiating time, delaying Brexit until Jan. 31 2020.

The government has said that it will both comply with this law and that Britain will leave the EU on Oct. 31 whatever happens. Johnson has not explained how he plans to take these two apparently contradictory steps. WHAT ABOUT A REFERENDUM?

Lawmakers who disagree with Johnson's approach to Brexit could use Saturday's debate to try to secure support for a second referendum on the decision to leave the EU. They could do this by making an amendment to whatever motion Johnson puts forward and calling a vote on it.

Labour is likely to vote for a second referendum if it is added to any deal proposed by Johnson, a party source said. While this would not be a binding commitment to hold a referendum, if a majority of lawmakers backed it, it would be hard to ignore and a big step forward for the long-running campaign for a second vote.

Other amendments are also possible. WHAT ARE THE NUMBERS?

The Prime minister needs at least 318 votes to be certain of victory in the 650-seat parliament. This number is less than 326 because seven Irish nationalist Sinn Fein party members do not sit, four speakers do not vote and four 'tellers' who help count votes, are not counted (although two of the tellers must support the deal and two must be against).

- Conservative Party - Johnson's party does not have a majority in parliament and is not united about the best plan on Brexit. There are currently 288 Conservative seats and most would be expected to vote with the prime minister. But there is a faction of committed Brexiteers that could rebel if they feel the deal does not provide a satisfactory break from the EU. The group, which can number as many as 80 but has a hardcore of around 28, are unlikely to vote as a single block and are difficult to predict.

The Democratic Unionist Party (DUP) - The DUP's 10 lawmakers are also pivotal to Johnson's chances of success. The party is allied with the Conservatives under a formal arrangement, but the DUP currently says it cannot support the Brexit deal. The DUP's final decision will also influence some of the Brexit-supporting Conservative's votes.

Conservative exiles - Johnson expelled 21 Conservatives from his party in September because they did not support his plan to leave the EU on Oct. 31, with or without a deal. Another, former cabinet minister Amber Rudd, quit the party over Brexit and also sits as an independent. Some could support his deal, others are more likely to reject it and back a delay to hold a second referendum.

Labour Party - Labour leader Jeremy Corbyn said his party cannot support the deal. "As it stands we cannot support this deal ... also it is unclear whether it has the support of his allies in the DUP, or indeed, many allies on his own backbenches," Corbyn said.

Asked whether he would put forward a motion of no-confidence to try to bring down Johnson on Saturday, Corbyn said the weekend was a time to discuss the Brexit deal and other issues would be for next week. Labour Rebels - Labour rebels are crucial to Johnson's hopes of getting a deal approved. A small handful of Labour lawmakers are explicitly pro-Brexit and have supported previous attempts to back a deal.

Another larger group of around 20 Labour rebels who want Britain to leave the EU with a deal could also back Johnson, depending on the final terms. Other Parties - Most remaining lawmakers are expected to vote against a deal. They are made up of 35 Scottish National Party members, 19 Liberal Democrats and 45 from smaller parties or independents. The SNP and Liberal Democrats said they opposed the deal. Some independents are likely to vote for it.

PREVIOUS VOTES Parliament has already rejected an earlier Brexit deal, negotiated by Johnson's predecessor Theresa May, three times.

Jan. 15 - Government lost by 230 votes when parliament voted 432-202 against the deal March 12 - Government lost by 149 votes when parliament voted 391-242 against the deal

March 29 - Government lost by 58 votes when parliament voted 344-286 against the deal

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