Brexit deal now hinges on Northern Irish unionists

On Thursday morning DUP issued a statement saying it could not support the agreement from the night before.

"We will continue to work with the government to try and get a sensible deal that works for Northern Ireland and protects the economic and constitutional integrity of the United Kingdom," they said.

The DUP also refused to back the agreement reached with the EU by the previous UK prime minister, Theresa May.

EU-27 leaders were planning examine the revised document, which sees changes to the so-called backstop: a mechanism to secure an open border on the island of Ireland, a key reason why the British parliament earlier voted against the deal three times.

If there was a deal, the EU-27 could give their political backing to the deal on Thursday.

But a "formal yes" could only come later, as the withdrawal agreement still needs to be approved by the European parliament. The British parliament will also vote on the divorce deal, possibly in an emergency sitting on Saturday.

An extension of a couple of weeks to the Brexit deadline of 31 October seems almost inevitable, even in case of deal, for the ratification and legal clarifications to take place, EU diplomats said on Wednesday.

"I cannot imagine leaders tomorrow being able to saying more [than] 'This doesn't look bad, let's continue to work with UK to finalise the details'," said one senior EU diplomat.

The revised backstop means that a regulatory and customs border would be drawn in the Irish Sea, but that Northern Ireland would legally remain in the UK customs territory - allowing Johnson to claim that the entire UK left the EU.

Complex details

Northern Ireland, however, would follow EU rules on tariffs and quotas with the exemption of personal goods between Northern Ireland and Great Britain, and a list of goods agreed jointly by the EU and the UK, that cannot be transported further to the EU, would be also exempt.

Another key stumbling block was the Northern Ireland assembly's say in the continuation of the backstop.

It would be set up in a way that four years after the end of the transition period at the end of 2020, the assembly at Stormont would be able to give its consent by simple majority, weakening the DUP's grasp of the process.

The vote would be repeated in four years - or eight years, if Northern Ireland's parties went for sixty percent threshold in voting for the continuation of the backstop, but failed. And if the assembly could not sit, which is the case currently, the backstop would continue.

The UK has also agreed to sign up to the EU's "level-playing field" provisions in the future trade agreement, promising not to abandon EU standards.

Also: Turkey, budget, enlargement

While on Brexit the EU-27 is united, leaders' will have to discuss difficult challenges that see deep divisions among member states on Thursday.

Turkey will be a focal point, along with its military offensive against US-allied Kurdish forces, as the EU agreed earlier this week to limit arms exports to Turkey.

EU leaders will also talk about their concerns over foreign Islamic State militants returning to Europe, as many of them excited from prisons as a result of the Turkish offensive.

Differences over opening the door to North Macedonia and Albania for EU accession talks also divide member states as France is determined to keep both countries waiting, while others are only concerned about Albania's possible membership talks.

The debate will centre around on the EU's functioning with more members and its geopolitical place amid rising threats from Russian and China.

Another hot topic will be the EU's long-term budget, which EU leaders will talk about in detail for the first time at this summit.

Leaders will be discussing the overall figure for the budget, with net payers wanting to stick to one percent of the bloc's gross national income, while others are arguing for the need for investment.

The rule of law conditionality, linking EU funds to the respect of the rules, and the EU's own revenue streams will also be on the agenda.

Leaders will also try to find agreement on climate targets ahead of the UN summit in December, but a detailed policy discussion can only be expected at the December summit of EU heads.