Brexit remains on a knife-edge this evening as the EU and UK try to hammer out a last-minute divorce deal ahead of a crunch summit tomorrow.
European leaders will meet in Brussels in less than 24 hours time for a showdown which has long been targeted as the moment at which a Brexit agreement will be finalised and signed off.
But with the two sides still locked in talks the get together could end up focusing on whether to delay Brexit beyond the current October 31 deadline.
If Mr Johnson is able to strike a deal with the bloc today it will put the UK on course for an orderly split from the bloc on Halloween.
But if he is unable to reach an agreement today it could trigger the most volatile two weeks in modern British political history.
Below is a breakdown of how things could play out.
What happens if Boris Johnson gets a deal today or tomorrow?
If the Prime Minister and the EU are able today to strike an accord acceptable to both sides then Mr Johnson’s next steps should be relatively straight forward.
The PM will attend a crunch summit in Brussels tomorrow and on Friday when European leaders will be asked to rubber-stamp the agreement.
They will almost certainly do so because no one nation will want to stand in the way of Brexit being resolved especially after such a long and arduous path to a deal.
Assuming the leaders of the EU’s 27 member states sign off the deal the PM will then have to present it to MPs for them to vote on.
That vote would likely take place on Saturday with the government having kept open the option of asking MPs to work at the weekend.
If Mr Johnson is able to win a vote on his deal then he will be almost home and dry.
Boris Johnson, pictured in Downing Street yesterday, is facing a race against time to strike a Brexit deal with the EU
Leo Varadkar, pictured in Dublin today, said he was ‘confident’ a deal could be done before October 31 but that there were still numerous hurdles standing in the way
Parliament would then spend the coming week putting in place the legislation needed to actually make Brexit happen on October 31.
If Mr Johnson has a majority at the first vote on Saturday there is no reason to think that it would evaporate when it comes to voting on the Withdrawal Agreement Bill.
Once Parliament has agreed the deal it will then be up to the European Parliament to do the same.
Brexit critics have raised the prospect of trying to block the deal but if the European Commission, European Council and British Parliament have all backed it, MEPs will be under immense pressure not to scupper the process.
Assuming all of those hurdles are cleared in a timely fashion then the UK will leave the EU with a deal on October 31.
What happens if talks continue but the deal has not been finalised and ratified by all sides by Halloween?
The EU is believed to be open to a short technical extension beyond October 31 to get the deal over the line.
But Downing Street remains adamant that there must be no Brexit delay in order to preserve Mr Johnson’s ‘do or die’ pledge.
However, if the two sides are close to a deal and an orderly divorce it is unlikely that the UK would pull the plug.
Ultimately, Mr Johnson may be able to accept a short delay if that is the price of getting Brexit done in a non-chaotic manner.
What about Saturday and what about a second referendum?
Remain-backing MPs are expected to try to force a vote on Saturday – assuming Parliament sits – on holding a second referendum.
The exact terms are not yet clear but it is thought that if there is a deal then they will agree to back it on the grounds that the public are then given a final say on whether to accept it.
That referendum would then likely pitch the PM’s deal against Remain.
Mr Johnson will resist any attempt to attach a second referendum to his deal and currently it is unclear whether there is a majority in Parliament in favour of a so-called ‘People’s Vote’.
If a majority does emerge for a second referendum it will be difficult for the PM to resist given that Remain-backing MPs have repeatedly shown a willingness to seize control of the Brexit process.
What happens if the PM doesn’t get a deal today or tomorrow?
If the two sides fail to agree terms today or tomorrow then the next 48 hours will become incredibly unpredictable.
Given that both sides have said progress is being made in the talks it is likely that those efforts would continue in a bid to get an agreement before Halloween.
But even if the talks do continue beyond today, the immediate focus of the EU and UK will shift to whether there will be a Brexit delay.
The first question will be whether Mr Johnson will attend the summit tomorrow. If there is no deal to be signed then European leaders will instead discuss pushing back the UK’s departure date.
The PM is unlikely to want to take part in those discussions because of his opposition to any delay which raises the prospect of him boycotting the summit.
What would happen on Saturday?
Regardless of what the EU 27 agrees, the Benn Act will legally require the PM to ask Brussels for an extension if there is no agreement in place by October 19 – this Saturday.
The government has said it will comply with the rebel law and that the PM will send a letter to the bloc asking for a delay.
However, Mr Johnson is also expected to make plain that he does not want that delay to be granted, putting the EU in a tricky position.
Number 10 has been locked in talks with the DUP in recent days over whether the unionist party can support a proposed deal. Arlene Foster and Nigel Dodds are pictured leaving the Cabinet Office today after a meeting with the PM
One way around writing the letter would be for the PM to ask MPs to vote in favour of a No Deal Brexit on Saturday.
But with a majority of MPs ardently against a disorderly divorce the PM would almost certainly lose such a vote.
A more likely outcome would be Remainer MPs forcing a vote on holding a second referendum.
It is currently unclear whether there is a majority in favour of a second national ballot on Brexit but it is thought the number of pro-People’s Vote MPs is growing.
If MPs were able to win a vote on holding a referendum they would then have to try to pass the legislation needed to make it happen in the face of fierce government opposition.
The other alternative is that if there is no deal to discuss then the government could opt to cancel the Saturday sitting.
What could happen next week?
Assuming that Mr Johnson complies with the Benn Act and asks for a delay and one is granted then next week could see a fresh vote on holding a snap general election.
Mr Johnson has already tried twice to go to the country early but was thwarted by anti-No Deal MPs.
Opposition leaders, including Jeremy Corbyn, have said that once No Deal is taken off the table they will support an early election.
That means that Mr Johnson, fresh from having to break his ‘do or die’ Brexit promise, could demand that Parliament finally let him have an election to break the stalemate.
If MPs say yes then polling day would be at the end of November or the start of December.
One alternative would be for the opposition to bring forward a vote of no confidence to oust Mr Johnson and then allow Remain MPs to try to form a government of national unity.