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?
Parliament will sit from 0830 GMT on Saturday Oct. 19 - the first time since April 3, 1982 when it discussed the Argentine invasion of the Falkland Islands.
Johnson will make a statement to lawmakers, followed by a 90-minute debate and then voting.
The vote would be intended to meet one part of the criteria for ratifying the exit deal. Legislation would then need to be passed by Oct. 31 in order to complete the ratification.
WHAT IS BEING DISCUSSED?
Johnson said he had agreed a "great" new Brexit deal. He is framing Saturday's vote as a choice between his deal or a no-deal exit. Others say the choice is more complex.
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 a vote on a deal and does not get approval for 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 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 (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 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 hard core of around 28, are unlikely to vote as a single bloc and are difficult to predict.
The Democratic Unionist Party (DUP): Its 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 says it cannot support the Brexit deal.
The DUP's decision is expected to influence some of the Brexit-supporting Conservatives' 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 has 244 members of parliament.
Leader Jeremy Corbyn said: "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."
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 number 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 the rest from smaller parties or independents. The SNP and Liberal Democrats said they opposed the deal. Some independents are likely to vote for it.
Parliament 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