Michel Barnier dramatically upped the stakes in the Brexit end game today as he set a midnight deadline for the EU and UK to have agreed a divorce deal.
EU leaders and Boris Johnson are due to meet in Luxembourg on Thursday for a crucial summit that will decide the future of Brexit. If they cannot agree a deal at the summit, the Prime Minister faces being forced by MPs to delay Brexit again.
Mr Barnier believes a blueprint must be agreed by close of play this evening if there is to be any chance of a deal being signed off by European leaders this week.
Downing Street has refused to sign up to the proposed cut-off point for a legal text to have been finalised before Luxembourg, as it said Britain would continue to work to get a deal done 'as soon as possible'.
If no accord can be struck within the next 48 hours then the focus of that summit in Brussels - the final one scheduled before the October 31 divorce date - will shift to whether to offer the UK a delay.
Below are all of the answers to all of the key questions as Brexit enters its most volatile phase yet.
When does Michel Barnier believe a deal needs to be done?
The EU's chief Brexit negotiator has reportedly told the bloc's 27 member states that the UK's latest proposals are not up to scratch.
Three diplomatic sources said Mr Barnier believed a legal text on a proposed way forward would need to be agreed by the end of talks today for him to recommend that the summit of European leaders on Thursday and Friday consider rubberstamping a deal.
Should the midnight deadline not be met then Mr Barnier is likely to recommend that more talks with the UK are needed, taking the two sides beyond the crunch summit.
That would likely mean the Brussels showdown focusing on whether to offer the UK a Brexit extension with more talks to follow in the run up to October 31.
How has the UK responded to this new deadline?
Downing Street has refused to commit to meeting it. The Prime Minister's official spokesman would only say that the UK wants a deal to be done 'as soon as possible'.
Asked if he recognised Mr Barnier's new cut-off point, the spokesman said: 'We are working hard. The Prime Minister is aware of the time constraints that we are under.
'We want to make progress towards securing a deal as soon as possible and we want to make progress ahead of the EU council on Thursday.'
Downing Street sources have previously hit out at 'artificial deadlines' set b by Brussels on the grounds that there is only one deadline that truly matters and that is the UK's Halloween departure date.
What is the mood surrounding the talks and how likely is a deal right now?
Both sides are still making plenty of positive noises about the prospects of a deal being done, if not this week then potentially by October 31.
However, both sides are also cautious about being overly optimistic because there are still massive gaps that need to be bridged.
Stephen Barclay, the Brexit Secretary, today made a last-minute trip to Luxembourg where EU ministers are meeting with Mr Barnier.
Speaking when he arrived, Mr Barclay said that talks needed 'space to proceed'.
He said 'detailed conversations are underway and a deal is still very possible'.
Ireland's deputy PM Simon Coveney echoed a similar sentiment as he said today that he remains hopeful a Brexit deal can be reached before the crucial EU summit.
Mr Coveney said: 'I don't think it's inevitable that they can't get a deal before the summit.
'I think what Michel Barnier said today was very clear, that it's difficult but possible to have a deal between the two negotiating teams this side of the leaders' summit.'
However, Mr Coveney sounded a cautious note as he said 'a lot of progress needs to be made today'.
'This isn't a time for optimism or pessimism quite frankly, we need to deal with the facts as we see them,' he added.
What is Boris Johnson doing to get a deal?
In terms of talks in Brussels, the PM is leaving things to his team of negotiators who have been trying to hammer out a way forward with their EU counterparts.
But Mr Johnson has been pressing his fellow European leaders to help him get a divorce deal and deliver on his 'do or die' Brexit pledge.
This morning he spoke to French President Emmanuel Macron for 20 minutes on the phone with Downing Street characterising the conversation as 'constructive' and a 'good discussion'.
More phone calls are expected. The PM has also tried to maximise the amount of time available for a deal to be done by postponing a meeting of the Cabinet.
Mr Johnson had been due to meet with his top team of ministers this morning but that meeting has been moved to tomorrow so that the government can respond to any last minute progress - or lack thereof - before the summit on Thursday and Friday.
What exactly are the two sides talking about and what are the sticking points?
Mr Johnson put forward a 'final offer' on how to replace the Irish border backstop earlier this month but the EU gave the proposals short shrift.
The PM is then believed to have compromised during a meeting with Leo Varadkar last week which resurrected the hopes of a deal being done.
Neither side has said in public exactly what has been put on the table by the PM, or where the EU has said it could budge, but it is thought the PM has put forward a so-called 'customs partnership' proposal.
There are two main sticking points: Customs checks and Northern Irish consent.
Mr Johnson's proposed way forward is thought to involve Northern Ireland leaving the EU's customs union along with the rest of the UK.
However, the bloc's tariffs would be collected on goods heading to the province from mainland Britain so that they are all EU compliant when they arrive on the island of Ireland.
If those goods then stayed in Northern Ireland - and within the UK - then the business receiving them would be eligible for a rebate on the EU tariff charged.
With checks having been carried out in the Irish Sea at ports there would be no need for border checks at the land border between Northern Ireland and the Republic and goods could head south with no impediment.
The EU is concerned about the complexity of the plan, the potential for smuggling, and whether technology exists to implement it.
The other sticking point relates to how the Northern Ireland Assembly can be given a say on what should happen to the border arrangements in the future.
The PM's original plan to hold a vote every four years hit problems on the technical feasibility of the proposals amid concerns that it would effectively give the DUP a veto.
A compromise is believed to be in the works but it is currently unclear exactly what it is.
Today it was claimed that Mr Johnson had compromised still further but both sides are keeping quiet and the details of any plan are only likely to be made public if and when a deal is finalised.
If Mr Johnson waters down his plan will it still get through the House of Commons?
The PM's original plans appeared to have the support of a majority of MPs.
But the premier is believed to have moved closer to the EU during the talks that have followed which means there are now major question marks over whether that level of support remains.
With no overall majority in parliament, Mr Johnson will be entirely reliant on the support from other parties, particularly his coalition partner the DUP, if his deal is to stand a chance.
The PM held talks with the DUP leadership on Monday in a bid to win them round.
The Northern Ireland unionist party is believed to have major concerns about the idea of a 'customs partnership' on the grounds it would see Ulster treated differently to the rest of the UK.
If Mr Johnson is unable to get the DUP on board with his deal then he will be in big trouble.
What will happen on Thursday and Friday?
EU leaders will meet in Brussels for the final European Council summit scheduled before the Brexit deadline.
The summit has long been targeted as the moment when a divorce deal would be finalised but that remains hanging in the balance.
If a deal is finalised in the next 48 hours then a deal could still be signed off by European leaders.
But if talks fail to reach a resolution then the summit is expected to focus instead on whether to offer the UK a Brexit delay.
If there is no agreement signed on Thursday or Friday could there still be a deal done before October 31?
Yes. It is possible and probably even likely that if progress is being made but no deal has been finalised by the summit that talks will continue in the days ahead in the hopes of a breakthrough before Halloween.
However, time will get tight incredibly quickly.
Drafting an agreement, translating it into the EU's different languages, checking it for legal problems, holding votes in the Commons and the European Parliament will all have to take place before a deal is done.
Downing Street believes it is doable. But the closer to October 31 we get after the summit, the harder it will be to get a deal done.
If an accord is in sight that could force the PM to accept a short technical extension beyond Halloween to get the deal over the line.
If no agreement has been struck by the end of this week's summit, both sides are believed to be open to holding a last minute emergency summit before October 31 to try one last time to agree a deal.
What does all of this mean for 'Super Saturday'?
The government has left open the possibility of MPs sitting on Saturday to discuss the fallout from the EU summit.
It would be the first Saturday sitting of Parliament since the Falklands War and while it is not guaranteed that it will go ahead, government sources said today that it is 'much more likely than not'.
If Mr Johnson has secured a deal with the EU at the summit then MPs will be asked to vote for it.
If he doesn't things could get a bit more complicated with a variety of potential outcomes in the mix.
The PM could ask MPs to back a No Deal Brexit - a vote he would almost certainly lose.
Regardless of if there is a deal to vote on MPs are expected to try to hijack proceedings to force a vote on holding a second referendum.
How likely is it that the push for a second referendum will succeed?
This is a tricky question to answer. It remains unclear whether there is a majority in the Commons in favour of a second public vote but it is thought many MPs are now moving towards that option.
If there is a majority then it should be relatively straight forward for MPs to force a vote and win it.
However, there is a potential stumbling block which could then arise.
If MPs vote for a second referendum and then try to pass a law to make it happen they will also need to somehow pass a money resolution to allocate the cash needed to organise another national ballot.
The difficulty for the rebels will be that it is only the government that can bring forward money resolutions.
That means Remain-backing MPs will probably have to change the rules in the Commons if they are to get their way.
Will Jeremy Corbyn support a second referendum?
Many people believe this will be the key to whether a push for a so-called 'People's Vote' succeeds.
Mr Corbyn is in favour of a second referendum taking place but only after a general election.
He is under pressure from his allies to change his stance and whip Labour MPs to vote for a second referendum. If he does then there could be a majority for the move.
What about the Benn Act?
The anti-No Deal law passed by rebel MPs will loom large on 'Super Saturday' if Mr Johnson does not secure a deal at this week's EU summit.
The law states that if no deal is in place by October 19 then the PM must ask Brussels to delay the UK's departure date to the end of January next year.
Mr Johnson has said he will comply with the law but he has also repeatedly made clear to the EU that he does not want a Brexit delay.
If MPs believe the PM has tried to frustrate the purpose of the law then the row is likely to end up in the courts.
Will the UK leave the EU on October 31?
Mr Johnson is adamant that he will stick to his 'do or die' promise to deliver Brexit by October 31 with or without a deal.
But there are so many moving parts and unknowns that it is currently impossible to give a definitive answer.
If the PM gets a deal with the EU this week then Brexit could and probably will happen on October 31.
If he doesn't and the EU offers an extension that the PM will be legally required under the Benn Act to accept then Brexit will almost certainly be delayed.
If there is no deal and no extension offered then the deadline will be met.