Will the UK stay in a customs union? Is Jacob Rees-Mogg about to become Prime Minister? Are civil servants working to undermine Brexit? The Tory civil war has more back-stabbing, front-stabbing and sub-plots than Game of Thrones.

Here’s the guide we all need to what is going on, and why.

What are they fighting about this time? 

It is, of course, all about Brexit. Forget for a second whether you are for or against the UK leaving the EU, and just put yourselves in the shoes of those who are desperate for it. Those who have spent their entire careers talking about it, campaigning for it, arguing in favour of it.

They were ridiculed, derided, ostracised by MPs not just on the opposite benches but those in their own party.

But in June 2016, they won the battle – a battle they never thought they would even get to fight.

Imagine the sense of relief, vindication and sheer joy.

But now imagine the worry it might be taken away. That hard-fought victory might not actually be a victory at all, as those charged with implementing the decision look to compromise at every opportunity.

There are three key strands to Brexit for those who campaigned for it. The UK taking back control of its money, its laws and its borders.

For the hardline Brexiteers, there can be no compromise on those goals.

But compromise seems to be on the horizon. The Government has already agreed to pay a divorce bill of up to £40billion to the EU. Theresa May and her Cabinet colleagues believe paying this bill is matter of honour for the UK, as the country should always make good on its commitments.

Then there is the so-called implementation period that May is calling for. This would see the UK continuing to obey EU rules for another two years after Brexit, but without any say in how they are made.

The die-hard Brexiteers were already struggling to swallow those pills before they latest row over whether the UK would completely leave the customs union once it quits the EU.

Why do I keep hearing about the ‘customs union’ all of a sudden?

Never has the difference between ‘a’ and ‘the’ been debated so intensely as when it comes to the UK’s post Brexit customs arrangements.

Quite simply, the European Union operates a customs union. This means the same tariff is charged on goods sold from countries outside the union to those within it. Animal products have an average tariff of 15.7%, sugars and confectionary have 23.6% and dairy has 35.4%.

Staying in the customs union would mean the UK would have to continue to abide by these tariffs, and therefore would not be able to negotiate trade deals with other countries. Brexiteers, therefore, want the UK out of it.

In her Florence speech last September, May gave them what they wanted when she said: “We will no longer be members of its single market or its customs union.”

However, not being part of “the” customs union is not the same as not being part of “a” customs union. In a document produced last summer, the Brexit department said the UK would seek one of two deals with the EU as part of the negotiations: A highly streamlined customs arrangement; or a new customs partnership with the EU.

Both of these options would entail some degree of mirroring what the EU’s customs union does.

To hard-line Brexiteers, this is unacceptable as it would restrict the UK’s negotiating goals.

But if this was all discussed last summer, why is everyone getting all rowdy about it now?

This issue has flared up in recent weeks after the business group the CBI called on the Government to stay in the customs union after Brexit until it becomes clear the UK can sign enough trade deals to make up for leaving it.

In the wake of this speech, Philip Hammond announced he hoped the UK to move “very modestly” away from EU rules and regulations. To Brexiteers, this was a sign that far from abandoning the red tape issued by Brussels, the UK would be still bound by it.

And what should happen a few days after this speech?

A civil service analysis is leaked showing that economic growth would be slower outside the EU – and the customs union – than if the UK stayed in.

The hard-line Brexiteers smelt a rat. They saw a direct line from the CBI speech to the leaked analysis. Rees-Mogg in particular was fuming, and believes the civil servants in Whitehall are trying to sabotage Brexit as they are institutionally against leaving the EU.

But far from having the Brexiteers in Cabinet ride to their rescue, backbench Tories were left isolated. International Trade Secretary Liam Fox said they would have to “live with disappointment” as the party’s slim grip on power meant there was little room for maneuver.  

The Brexiteer anger could seemingly be heard halfway across the world. Despite being in China on trip to promote British trade with the Far East powerhouse, Fox decided it was important to hit back against accusations he was selling out on his former Eurosceptic allies. He told reporters: “It is very difficult to see how being in a customs union is compatible with having an independent trade policy.”

Note the use of the word “a” – not “the”. Fox was breaking with what the Government’s position of some alignment post-Brexit.

After attacking Hammond, civil servants and the very validity of economic forecasts, the hard-line Brexiteers seem to have moved one of its most prominent generals into shifting position.

What about Theresa May? Surely she should be sticking by what she said before?  

Theresa May has been criticised by both sides of the Brexit divide for not setting out what it is she wants from the EU negotiations. Both sides believe it is because she will give in to the opposite point of view. This weekend, it seemed the Brexiteers scored a major victory when Downing Street seemed to clarify the UK would be leaving the customs union after Brexit.

“It is not our policy to stay in a Customs Union,” a Downing Street source said.

The shift from “the” to “a” reached the top of Government.

So does this mean the battle is over?

Absolutely not. This week May will chair two meetings of her Brexit ‘war cabinet’, to discuss the UK’s future customs deal with the EU.

The options on the table are a highly streamlined customs arrangement; or a new customs partnership with the EU.

Exactly the same as those put forward last summer.

The only thing that has changed is the introduction of the word “stay” into the conversation. The UK will not “stay” in a customs union. But Downing Street has not ruled out copying aspects of the current situation, rebadging it, and claiming it is brand-new customs arrangement.

The hard-line Brexiteers won’t be happy with that, will they?

No, but the problem is, and as much as they hate to admit it, there is not a majority in the Commons to support their vision of life outside the EU.

Anna Soubry, the most vocal of all the Tory anti-Brexiteers, claimed last week there is only about 35 hard-line backbenchers as she effectively accused the Prime Minister of letting the tail wag the dog.

Additionally, and again as much as the Brexiteers might not like it, the numbers are the numbers and the forecasts are the forecasts. If ministers are being given information by civil servants flagging up the economic hit which completely leaving the customs union would cause, they have a duty to take that into consideration.

There is no doubt the sense of Brexit betrayal is strong with a small but powerful group of Tory MPs. If they wished, they could orchestrate the removal of Theresa May, but there is no guarantee they would be able install their preferred successor in Downing Street. Indeed, such a hostile act may alienate many of the middle-of-the-road Tories who just want Brexit over with, and they could scupper the grand plans of Rees-Mogg and his allies in a pique of fury.

Theresa May clearly thinks that appeasement is the best way to stave off a full-bloodied conflict – something Tory leaders have tried before. And look how that ended.  

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Who’s Who in The Tory Civil War…

Jacob Rees-Mogg

The MP elevated to hard-Brexiteer-in-chief is Jacob Rees-Mogg. With his double-breasted suits, old-fashioned demeanour and, some might say, outdated social attitudes, he is jokingly referred to as ‘the MP for the 1950s’ by political comedians.

But his growing power in Parliament is no laughing matter. He is the chairman of the European Research Group – a band of Tory MPs who pressure the Government to deliver a full fat Brexit. No one is sure precisely how many Tories are part of the group, but estimates range from 50 to 80 and records show current Cabinet ministers have poured money into its coffers.

Theresa May 

The Prime Minister is the seeming living embodiment of the notion of being in office but not in power. Despite holding the highest elected office in the land, she seems unable – or unwilling – to make a firm decision on what the UK’s future relationship with the EU should look like.

Having backed Remain in the referendum, May seemed desperate to reassure Eurosceptics she would deliver the kind of Brexit they desired, and promised to take the UK out of the Single Market and look again at Customs Union membership in her Lancaster House speech in January 2017.

Yet her plan for a two-year transition period after March 2019 on the same rules as the UK’s current EU membership have riled up Brexiteers, who are scared of betrayal.

David Davis

The man who came in from the cold, it seemed David Davis’ frontbench career was over until Theresa May appointed him Brexit Secretary in July 2016.

Despite being heralded by Brexiteers as one of their own when he was given the job, he has started to depart somewhat from the hardline ethos of the Rees-Mogg crowd. Davis is fully behind the two-year implementation period, even if that means EU law will continue to apply after March 2019.

Boris Johnson

The front-man of the official Leave campaign, Boris Johnson is desperate to make sure the cause he so controversially supported is delivered in full and is a success.

Ahead of last year’s Tory conference the Foreign Secretary made a flurry of interventions, saying the transition period should not last “a second longer” than two years, and current rules on freedom of movement should end in March 2019.

Philip Hammond

The constant attacks on the Chancellor by hard-line Brexiteers show just how despised he is. As Foreign Secretary before the referendum, Hammond talked of “lighting a fire under the EU”, but now it seems he wants calm waters between the UK and Brussels.

Seen as wanting to keep the UK as close to the Single Market and customs union as possible, the latest round of Tory bickering really kicked off when Hammond suggested Britain would only moved “very modestly” away from EU rules after Brexit.

Liam Fox

The International Trade Secretary jokes he runs the “Sunshine Department”, as his job is deliver all the good things which Brexit is supposed to bring.

In recent weeks there have been murmurings that he is turning his back on his old Brexit allies, but the truth is he is one of the most free-market loving Tories in the Cabinet.

He seems to have taken a pragmatic position on the two-year transition period, saying that having waited 40 years for Brexit, he can stomach an extra 24 months if needs be.

Michael Gove

Banished to the backbenchers after stabbing Boris Johnson in the back during his ill-fated campaign to be Tory leader, Michael Gove is now one of the most popular ministers in Government.

The Environment Secretary is seen to be more pragmatic than ideological when it comes to Brexit, and understands the Government has to take the 48% who voted Remain with it as much as the 52% who backed Leave.

He has already won the argument over keeping animal welfare rules as tight as they are now – seemingly beating International Trade Secretary Liam Fox in the Battle of Chlorinated Chicken.

Anna Soubry

The former Business Minister has become the Tories’ most outspoken critic of the Government’s Brexit policies. From her position in the corner of the Commons on the backbenches, Soubry takes any opportunity available to urge the Government to keep the UK inside the Single Market and Customs Union – a move which Brexiteers say would not be respecting the referendum result.

Despite receiving regular heckling from her own side, Soubry gives as good as she gets, and her interventions are cheered by Labour backbenchers.