Boris Johnson is off to Brussels and hopes are high that today could, finally, be the day a post-Brexit trade deal is struck. 

The prime minister will meet EU Commission president Ursula von der Leyen for dinner tonight in a last-ditch bid to bridge the gap between the UK and EU. 

It follows troubled and lengthy talks breaking down on Monday, spreading panic among industry leaders with the transition period due to end on December 31. 

Government sources have underlined ministers “do not consider this process to be closed” but stressed “political impetus” is now needed to get an agreement over the line. 

So, as the stage is set, will this the last act of Brexit theatre? Or is the long-running saga destined for yet another sequel? 

Here’s everything you need to know. 

Why haven’t they done a deal already?

This is a good question. The answer is not simple. Nor is it, like so many other crises in 2020, ‘because of the coronavirus’. 

EU chief negotiator Michel Barnier, his British counterpart David Frost and their respective teams have been in regular contact throughout the pandemic. 

They remain bitterly divided on three critical issues: fishing quotas in UK seas, rules governing fair and open competition (often referred to as ‘level playing field’ regulations) and how to resolve disputes. 

Critics of Johnson’s strategy also argue the PM did not make it easy for British diplomats by tabling the Internal Market Bill in the Commons earlier this year.

Some powers originally in the legislation, if used, would have broken international law as they ripped up parts of the withdrawal agreement Johnson signed off in 2019 relating to Northern Ireland. 

The move soured relations and prompted Brussels to start legal action. 

In the strongest sign yet that a deal is in the offing, however, Michael Gove and European Commission vice-president, Maros Sefcovic, struck a deal on Brexit divorce terms and the controversial clauses were removed from the legislation. 

If they do a trade deal, is it immediately sorted? 

Not quite. This is Brexit we are talking about. 

If the prime minister and von der Leyen do back an outline agreement – and government sources said on Tuesday night “we must be realistic” that “an agreement may not be possible” – there are still a few hurdles for both sides to clear.

The deal must be ratified by both sides.

For the EU, that means the European Council of Ministers, which brings together heads of state from the 27 members, giving it their political blessing. 

They are due to meet over two days on Thursday and Friday. 

Later in December, MEPs in the European Parliament must also vote for the deal, but this is almost a given if it is given the green light by ministers. 

Johnson, meanwhile, must sell the deal to his Cabinet and MPs in the Commons are expected to be given a vote. 

Reports at the weekend suggest Johnson will easily win the support of his ministers, many of who witnessed at close quarters how Brexit destroyed the authority of both David Cameron and Theresa May. 

What about Tory Eurosceptics? And Labour MPs? 

After riding to a stunning election victory in 2019 on a promise to ‘get Brexit done’, Johnson has an 80-seat majority in the Commons. 

Given the alternative is no deal at all (more on this later) and many new Tory MPs owe their political career to Johnson, any deal he brings back is unlikely to hit a serious stumbling block

But it is by no means guaranteed to clear the Commons either. 

Eurosceptic backbenchers, who haunted May’s time in office, could prove particularly tricky to win over. 

The European Research Group of hardline Tory Brexiteers, who complained they were “bounced” into backing the withdrawal agreement, may demand more time and independent legal advice. 

MP Bill Cash, a veteran of the cause, issued a shot across the bows to ministers on Tuesday, after news emerged of Gove’s Northern Ireland Brexit divorce deal.

He warned Johnson MPs will be “watching” with “great diligence” what he brings back from Brussels. 

He said: “It’s about democracy, it’s about freedom, it’s what Churchill was proud of, it’s what Margaret Thatcher was proud of, it’s what we’re proud of – and I simply make this final point, we will maintain our sovereignty at any price.”

While Johnson has a lot of breathing space, if the deal sends shockwaves through his MPs he could be left leaning on Labour MPs’ support. 

Keir Starmer is also awaiting for any deal which emerges but is minded to back a deal, according to reports. 

He is also under pressure, however, as many pro-EU backbenchers, and some of his shadow cabinet, remain divided over whether to support an agreement or abstain.

So, if there is a deal will much change? 

Any deal is anticipated to see Britain leave the EU’s single market and customs union on December 31. 

From January 1, the new trade and security deal will be in place and Britain will begin to operate its points-based immigration system.  

If it is a “thin” trade deal, it could be built on in early 2021. 

Whatever deal is made, there will be some, perhaps a lot, of disruption at the ports, borders, in the markets and in industry as the UK adjusts to a new set of regulations.

There will be a hit to the economy. How significant this will be to a country already grappling with Covid-19 is not yet clear. 

What if there is no deal? 

The vast majority of experts, economists, politicians and businesses owners regard this outcome as a catastrophe. 

It would mean that from January 1, the UK will have no formal trading relationship with the EU and would be treated as any third country outside of the bloc.

A leaked Whitehall document, known as the Yellowhammer report, predicted a three-month “meltdown” as rules and regulations governing imports, exports and customs would effectively be ripped up overnight under no-deal. 

The UK would fall on to World Trade Organisation rules and there would be significant tariffs on trade on goods and manufacturing and agricultural exports.  This would in turn make many businesses uncompetitive, jeopardising jobs and industry. 

The UK would have a hard border with the EU, including in Northern Ireland.  

Officials forecast lines of 7,000 trucks at Dover and two-day waits to get into France. 

This could also mean delays to food supplies, goods and potentially medicine. Food prices would go up as a result, ministers admit. 

The Office for Budget Responsibility (OBR) warned earlier this month that no-deal would leave the UK worse off by a further 1.75% of GDP (gross domestic product) next year and unemployment could reach 8%. 

To put that in context over time, Bank of England governor Andrew Bailey has said that no-deal would be a greater long-term hit to the economy than the coronavirus pandemic. 

A report on Sunday also suggested supplies of the Covid-19 vaccine would have to be flown into the UK by military aircraft.