What caused today’s Brexit crisis?

The Democratic Unionist Party said it “will not accept” a deal on the Irish border question brokered by Theresa May which treats Northern Ireland differently to the rest of the UK, throwing the already difficult talks into chaos.

The DUP is effectively propping up Ms May’s minority Government, giving the group of just 10 Irish MPs a significant influence on UK politics.

DUP leader Arlene Foster said: “We have been very clear. Northern Ireland must leave the EU on the same terms as the rest of the United Kingdom,” she said, speaking at Stormont.

What was supposed to happen?

Theresa May met Jean Claude Junker, the President of the European Commission, in Brussels for the latest round of Brexit negotiations.

A deal on the Irish border question has to be reached as a matter of urgency, as when the UK leaves the EU in March 2019, the Republic of Ireland will remain a member while Northern Ireland, as part of the UK, will be outside of it – an unprecedented situation because the Irish Republic and the UK joined the union at the same time in 1973.

The hope was that a draft proposal would have been agreed allowing Northern Ireland to maintain “regulatory alignment” with the EU, preventing a hard border with the Irish Republic.

What was the sticking point?

Ms Foster gave a fiery press conference, declaring any such deal had not been approved by her party – regarded as hardline unionists.

She said: “The Republic of Ireland government is trying to unilaterally change the Belfast agreement without our impact and without our consent.

“Of course we will not stand for that.”

Within 20 minutes of this conference, Ms May is understood to have halted the talks to make an emergency call to Ms Foster.

After a three and a half hour working lunch, the Prime Minister and Mr Juncker emerged to admit in a slightly awkward press conference they had reached a stalemate in the talks.

What does “regulatory alignment” actually mean?

Regulatory alignment could mean both Ireland and Northern Ireland following the same rules governing trade, to ensure that goods can continue to move freely across a “soft” border with no customs checks.

But critics say this would effectively move the customs border between the UK and the Republic into the Irish Sea. This alarms the Unionists as they say it would mean Northern Ireland being treated as though it is part of a united Ireland instead as part of the UK.

The border between the North and the Republic of Ireland has never been a hard one, but the proposal to keep things as they are in terms of free movement of both goods and people would create a “backdoor” between the EU and the UK – for immigrants as well as goods.

What other implications would “regulatory alignment” have for a post-Brexit UK?

Leaders of the devolved governments in Scotland and Wales as well as London Mayor Sadiq Khan seized upon the possibility of separate trading arrangements for different areas of the UK.

Scottish First Minister Nicola Sturgeon, a strident Brexit opponent, said: “If one part of the UK can retain regulatory alignment with the EU and effectively stay in the single market – which is the right solution for Northern Ireland – there is surely no good practical reason why others can’t.”

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And Welsh First Minister Carwyn Jones said: “We cannot allow different parts of the UK to be more favourably treated than others.

"If one part of the UK is granted continued participation in the single market and customs union, then we fully expect to be made the same offer."

Mr Khan said the deal being discussed in Brussels would have "huge ramifications for London", which voted overwhelmingly to remain in the EU.

"If Theresa May has conceded that it's possible for part of the UK to remain within the single market and customs union after Brexit … and a similar deal here could protect tens of thousands of jobs."

What will happen next?

Pro-remain MPs, who now favour a “soft” Brexit, have seized on the sticking point over the Irish border question to say the simplest solution would be for the UK to remain in the single market and customs union – which would entail paying into the EU budget and accepting continued freedom of movement – an absolute red line for most Brexit supporters.

Irish Prime Minister Leo Varadkar has said he is prepared to give Ms May more time to reach an agreement.

Speaking in Dublin, he said the British and Irish governments had agreed the draft wording of a deal on the Irish border on Monday morning, and that he was "surprised and disappointed" it had been pulled from the table at the eleventh hour.

Ms May said the UK and the EU would "reconvene before the end of the week".

Is the peace process under threat?

While there is no immediate threat of a return to the troubles, the political situation in Northern Ireland remains delicate. Leading Brexiteer Jacob Rees-Mogg accused Dublin of “promoting the creation of a united Ireland” by trying to force Britain’s hand on the border issue.

Mr Juncker insisted there was “a common understanding on most relative issues”, while Ms May said the meeting had been “constructive”.

Mr Varadkar said that while it is important to listen to the views of the DUP, it is also crucial to bear in mind the position of the other northern Irish parties.

Irish nationalist parties, including the SDLP, favour the whole of the UK remaining in the customs union and the single market.

Mr Varadkar insisted there is no “hidden agenda”, and said the soft border solution complies fully with the terms of the Good Friday agreement, which sealed the peace process in 1998.

Could this mean the beginning of the end of Theresa May’s premiership?

Pundits have been predicting Ms May will not last the course of the Brexit negotiations ever since she lost her majority at the June general election, having to rely on a £1bn agreement with the DUP to cling onto power.

This tenuous “confidence and supply” arrangement means the unionist party votes with the Government on all crucial matters of budget and finance, allowing the Prime Minister a slim working majority to get bills passed through the House of Commons.

The deal brokered between the Conservatives and the DUP was much criticised by opposition parties at the time, with pundits predicting there would be trouble ahead.

Labour leader Jeremy Corbyn said after the breakdown: “The real reason for today’s failure is the grubby deal the Government did with the DUP after the election."

Vince Cable, the Liberal Democrat leader, said: “The DUP must not be allowed to dictate the UK’s Brexit negotiations. This again shows the Conservatives are in office but not in power.”