The Irish border is now central to the civil war tearing the Tory Party asunder.

The pro-Remain camp sees the issue of Britain entering a customs union as their best reverse gear. And a major rationale is that it would help avoid a hard border in Ireland.

Brexiteers see any talk of remaining in the customs union, or a customs union, as a betrayal of the Brexit vote.

Fabian Zuleeg, chief executive of the European Policy Centre in Brussels, makes the point that for Brexiteers, to abandon the customs union would remove that last economic argument for leaving the EU - the idea that Britain would have its own independent trade policy - so they will go down fighting, and take British Prime Minister Theresa May with them.

"The referendum vote was clear," declared leading Brexiteer Michael Gove. "We need to take back control of trade - that means leaving the protectionist customs union."

During a non-binding motion on remaining in a customs union, the pro-European Tory Dominic Grieve told the Commons that it was the very least Britain needed to do: "This 'customs union bad, somehow free-trade agreement good' simply does not stack up. It is time for a reality check. In fact, we need more than a customs union because, as is also clearly obvious, we are not going to be able to trade without regulatory alignment."

On Thursday the pro-Remain Home Secretary Amber Rudd found herself clinging to her portfolio when she said at a press lunch that the issue was still under discussion in cabinet.

The Daily Mail and the Sun weighed in, with the Mail warning that remaining in the customs union would be "an utter humiliation," while the Sun said that it would "hold" Theresa May to her promise that Britain would leave the customs union.

Downing Street was prompted to (once again) clarify.

We are leaving the customs union," a spokesman said. "We will have an independent trade policy and we will strike trade deals around the world."

In the Seanad this week, Ireland’s EU Commissioner Phil Hogan accused Brexiteers of wanting to continue the conflict "until there’s not a building left standing on the other side".

He denounced the idea that Britain could make up for lost trade with the EU by striking free trade deals on its own with the rest of the world.

"Global Britain is stepping out of the huge network of global trade deals that the EU has negotiated and into a difficult world," he said.

"It is legitimate, therefore, to doubt whether it can achieve more trade for the UK than at present, at least on any realistic time scale. On optimistic assumptions, even UK civil servants say it can’t," he said.

The latest convulsions began with last week’s front page splash by the Daily Telegraph, that the EU had rejected the UK’s ideas on avoiding a hard border and that, as such, remaining in the customs union was now back on the table.

The Sunday Times then reported that cabinet ultras Boris Johnson and Liam Fox would resign if the Prime Minister caved on the issue, but David Davis and Michael Gove might not.

That fuelled more bloodletting. The cabinet truce brokered by Mrs May in Chequers in February was over.

Even by the standards of the long Brexit war, it’s been a gory week.

This was all somewhat unexpected. The Labour Party had seized the initiative on the customs union issue, so it made no sense for the Tories to hand Jeremy Corbyn the initiative ahead of the local elections.

How did things blow up?

In March, EU leaders gave a provisional green light to a two-year transition period beyond March 2019, and to the start of parallel talks about the shape of a future trade relationship.

The proviso was that the UK would have to abide by its commitments on the Irish border.

A schedule of meetings between EU and UK officials was drawn up. Having acknowledged in a letter to the European Council President Donald Tusk that the Irish backstop, or Option C, would form part of the Withdrawal Agreement, Theresa May seemed to have conceded on the need for a legal text that "operationalised" it, despite having vehemently dismissed it in February as something no British prime minister would ever agree to.

Between the European Council and Wednesday 18 April, those meetings came and went and had little to show for it.

London’s preference has always been to avoid the backstop altogether and solve the border issue through the future trade and relationship.

When pressed, London has suggested a radical new customs "partnership" and the use of technology. These suggestions were fleshed out in two UK position papers put forward - and brusquely dismissed by the EU - as far back as last August.

Theresa May revived them in her Mansion House speech on 2 March.

The ideas can be boiled down into options. A customs partnership and what’s called "maximum facilitation", or MaxFac.

The "partnership" idea would mean at the UK’s border, British customs officials would mirror the EU’s rules for imports from the rest of the world. These officials would apply the same tariffs and rules of origin as the EU for those goods arriving in the UK but intended for the EU, with a "mechanism" that the UK would also apply its own tariffs for goods intended for the UK only.

The UK would then forward the difference to Brussels by tracking goods as they transited on to where they were supposed to go, i.e. the single market.

This would mean no checks at the Irish border, as the British customs officials would know what goods were destined for the UK only (and subject to the UK tariff) and what goods were destined for the EU (via Ireland and subject to the British-administered EU tariff).

Augmenting this would be further exemptions for cross-border trade. Entry and exit declarations would be waived for goods moving north and south. Goods moving from the UK and the rest of the world through the EU would not pay EU duties.

There would be a "trusted traders" scheme for larger operations, augmented by technology to reduce delays at ports and airports. Both sides would "commit" to reducing the cost and burden of complying, and SMEs, which, according to Theresa May’s speech, made up 80pc of cross-border traffic, would be simply exempt from EU customs requirements.

Irish and EU officials were left scratching their heads after listening to the Mansion House speech.

"These two options have been on the table a long time," says one source. "Theresa May went back to them in the Mansion House, but we saw that as the UK not having any new ideas. We know that they know that it’s just not doable."

"It was probably the dead-end we were always going to get to," says a government source.

In his Meath Chronicle article two days after the speech, former Taoiseach John Bruton was much more scathing of the "partnership" idea.

"The scope for abuse, and exchanging of goods, seems to be unlimited here," he wrote. "Consignments could be substituted for one another, and there would be no check on them when they crossed the Irish border. Such an arrangement would [be] very difficult to police, and is unlikely to satisfy the EU Customs Code."

On Wednsday 18 April, the two main negotiators, Olly Robbins on the British side, and Sabine Weyand on the EU side, met in Brussels to assess progress in the Irish technical discussions so far (the meeting was preceded by an informal encounter involving John Callinan, Ireland’s chief negotiator).

Ms Weyand, a straight-talking German official, is believed to have made it clear to Mr Robbins that the British suggestions were not going to fly. The following day, Stéphanie Riso, principal adviser to Michel Barnier, briefed officials from all 27 member states who comprise the Article 50 Working Party.

The equally straight-talking Ms Riso had the same message.

"The rejection of the British approach under these two proposals was laid out fairly firmly," says one source. "The Task Force view was that there was a fair amount of time-wasting. We could go on for a long time talking about options that realistically were never likely to be viable."

Irish officials looked on with alarm. In December London had agreed to the "backstop" of Northern Ireland remaining in the customs union if other solutions couldn’t be found. When that backstop was given legal effect in the first draft of the Withdrawal Treaty in February it was rejected out of hand by Theresa May.

Fine, said Irish and Task Force officials, you don’t like it. Then produce an alternative.

Nearly five months since the December deal, Dublin and Brussels viewed that alternative as London simply reheating what had already been rejected.

On Wednesday, however, the Brexit Secretary David Davis gave a bullish exposition to the House of Commons Committee on Exiting the EU.

Britain’s ideas had not been rejected, he said. This was simply the Commission’s opening position.

The technology on reducing queues and delays at borders was already out there, he said. "I was talking to one of the industry groups [in Northern Ireland] about their requirements, [how] to supply the UK through the Irish Republic. Much of it exists, but it’s the question of the extent of it [that] is used.

"APNR [Automated Number Plate Recognition] is already used in terms of tracking big shipments. An Approved Economic Operator [ie, trusted trader] system exists, but not in sufficient quantity to make it work. Electronnic pre-notification exists at the moment. A great deal of work on compliance on excise [duties] is done electronically, with pre-or post-notification."

Maximum facilitation, he said, "takes all the best practice around the world and puts it into one [system]".

Throughout the 90-minute hearing, Mr Davis repeatedly downplayed the need for a backstop solution in the Withdrawal Agreement (indeed, at one point he agreed with the DUP’s Sammy Wilson that if the EU "insisted" on a backstop it still wouldn’t require Northern Ireland to be treated any differently from the rest of the UK, and that the rest of the UK would not be fully aligning with the EU’s body of law on the customs union and single market).

Instead Mr Davis places the burden of solving the border on the future trading relationship, in other words, the Option A solution to avoiding a hard border.

This, through his evidence to MPs, is his essential argument:

A comprehensive and generous free trade agreement (FTA), which abolishes tariffs and quotas is certain, because both the EU and UK want it.

You also get an agreement on "mutual recognition" of standards and regulations. Then you get agreement on how to deal with rules of origin (which will affect supply chains snaking back and forth between the EU and UK).

When pressed by Stephen Kinnock MP that these are not part of the Withdrawal Agreement, and that the Irish Protocol must be part of that agreement, and concluded by October, Mr Davis switched his emphasis to the political declaration which will accompany the Withdrawal Agreement and will look forward to a future FTA.

The political declaration, said Mr Davis, will have to be substantive (a word he used repeatedly). Get as much detail as possible on the key ideas - a customs "partnership", mutual recognition of standards and regulations, new technology under the MaxFac plan - and then get agreement in principle on all these things, and get it into the political declaration.

"Once you have got the ‘in principle agreement’…" Mr Davis told MPs, "…that’s the point at which we’ll turn around and say, on that basis Option A will work like this, and we’ll be working on that from now."

In other words, to those who complain that the UK is not providing a credible solution on the backstop, don’t worry: it will all be agreed in principle in the political declaration, and then the technical details will be worked out during the transition.

"The reason for the implementation [transition] period," he explained, "is to allow us [to move] from 'substantive', to 'agreed details' before it starts. Then during the implementation period, [we have] ratification and practical effect."

When he was pressed about the time in which this could be done, Mr Davis responded: "Don’t think we’ll wait till it’s done. We’ve already got work on this."

Mr Davis again placed great faith in the EU’s desire to get an FTA. "Go back to the [European] Council [negotiating] guidelines," he told MPs. "They want tariff free arrangements, which is the key stone in the arch of this. That is the overwhelmingly probable outcome, so I don’t think Options B and C [ie, the backstop] are at all likely - or needed."

The arguments against the Davis narrative are immediate.

The EU does not believe that a blanket concept of "mutual recognition" of standards and regulation can exist with the UK outside the institutional framework of the EU, where there are harmonised rules, monitoring, enforcement and ECJ arbitration.

The political declaration will not be as "substantive" as Mr Davis expects.

The Irish backstop will, according to Michel Barnier’s mandate, have to be given binding legal effect in the Withdrawal Treaty, and not waived away in favour of the promise of a dubious sequence of virtuous and speedy events.

Most importantly, Brussels and Dublin suspect that the UK wants to use the Irish border issue to accelerate the principles and promises of an FTA in such a way as to optimise the outcome for London - the Trojan Horse theory.

"The UK makes a pitch on Ireland," says one EU source, "and they then want to use that as a test bed on other stuff. This is radioactive for the EU in terms of other areas of the single market [where the UK wants concessions], such as on financial services."

Another source confirms the EU view that London wants to leverage the future relationship using Ireland as "a back door".

What also alarms Dublin is the not-so-subtle threat that spiced Mr Davis’s intellectual argument.

In effect, he was arguing that it was in Ireland’s economic interests to favour a highly generous FTA over the backstop. Everybody, including Leo Varadkar and Simon Coveney, really wanted an FTA.

"We view the circumstances of Northern Ireland and the protection of the Good Friday Agreement, and indeed the protection of our closest neighbour - it’s the economy of the Republic of Ireland [which is] as fundamentally important. But it doesn’t mean the best route to a solution is via Option C [the backstop].

"We all of us think," continued Mr Davis, "including the Irish Government, that the best option is Option A, and of course Option A hangs on how comprehensive a free trade agreement you have. If you don’t have a tariff or a quota between ourselves and any of the [EU] 27, including the Republic of Ireland, then that makes all of the customs agreement options easier…

"It will also protect both the Northern Ireland and the Republic of Ireland’s markets and economic position.

"There’s a massive amount of east-west [trade, between the UK and Ireland]. You’re talking about a billion a week… and nearly all of it is east-west.

"But even some of the North-South, or the Northern Ireland back to the mainland of Great Britain [trade] is through the South as well. One of the primary routes [for exporters] is Belfast to Dublin, across the Irish Sea to Holyhead and then to the south of England. It’s a trip which starts at 3pm and has to be concluded between two and four the following morning."

The other implied threat was over how substantive the political declaration is. Here, Mr Davis exhumes 2017’s notion that the UK divorce bill is a quid pro quo, not a basic accounting requirement, as the EU sees it.

"What we will have in place by October is going to be substantive. It has to be substantive - especially from the British parliament’s point of view. The British parliament is going to be voting on the Withdrawal Agreement. It will be voting for a [exit] bill of £35-39bn. People will want to know what we are getting in exchange. The hardest time I will have in October is when people say, ‘what have we got for this?’"

None of these arguments are going down well in Dublin.

"Even if in the meantime or subsequent period [after the Withdrawal Agreement], better options do crystalise, fine," says one government source. "The British argument is, we don’t like the backstop so therefore we don’t ever want to have to invoke it.

"What we’re saying is that given the timeframe we’re under, the answer to that can’t be to kick the whole thing down the road."

While the torment over the customs union, and the civil war in the Conservative Party, is an interesting spectacle for Ireland and the EU, there is frustration that the elevation of the issue into a make or break thing for the border misses the point.

The UK staying in a customs union would only go part of the way in avoiding border checks.

"We need a whole panoply of things in order to avoid a hard border on the island of Ireland," says one source. "That’s not just the customs union and regulatory alignment, but also some kind of a deal on how to supervise and enforce this stuff."

But the internecine warfare could topple Mrs May’s government.

Her customs "partnership" idea has been rejected by the EU on the grounds that it is technically and legally dubious, that it would place a burden on the EU to chase up the outstanding tariffs from the UK, and that it could be illegal according to WTO rules (if Indian exporters send goods to the EU in this scenario, they would be paying two different tariffs; would the lower UK tariff be seen by the WTO as a subsidy?).

More worryingly from her point of view, the idea has been shot down by hardliners in her own party, as it would still leave the UK entangled with Brussels.

Writing in Conservative Home, the Tory eurosceptic MP Bernard Jenkin concluded: "Even if the idea did work, it would surely still require either a regulatory border for UK products not made to EU standards or UK conformity with EU laws, while importing goods that are not. So why would this serve any better than a standard customs frontier?"

Meanwhile, the clock is ticking.

EU officials believe the UK has been time-wasting, and that a legal text has little scope for fudge.

Furthermore, the EU’s own rules governing its customs union are strict, as are the WTO’s own understanding of how a customs union is underpinned legally.

"It’s getting binary at this stage," says one source.

The Irish Government says it is critical that the UK makes substantial progress on the backstop by June.

The view in Dublin is that six months after the backstop idea - Northern Ireland remaining in the customs union and parts of the single market in order to avoid a hard border - first emerged, London has not yet left the starting gate.