It was believed that a deal had been made for Northern Ireland to maintain 'regulatory convergence' in a post-Brexit era, which would have put customs regulations and trade between Ireland, North and South, on a similar footing.

However, DUP leader Arlene Foster stated emphatically during a press conference in Stormont that her party would not support any agreement which would set Northern Ireland apart from the UK economically.

Following this, President Juncker and Prime Minister Theresa May had a "friendly and constructive meeting", before holding joint press conference where Juncker called May a "tough negotiator" and said that they had failed to reach "a complete agreement". It is widely believed that the DUP, who have a confidence and supply agreement which props up the Tory government in Westminster, are responsible for scuppering an agreement.

Hearing it was the DUP call that sunk today's chances of a deal - Foster held her press conf, 20 mins later May leaves talks with Juncker to call her, goes back into the room and the deal is off

— Laura Kuenssberg (@bbclaurak) December 4, 2017

Read both Juncker and May's statements in full.

President Juncker:

We had a friendly and constructive meeting today. It’s always a pleasure to meet Prime Minister May because our present relations is one of the most excellent which exists for the European Union.

But I have to say that she is a tough negotiator, and not an easy one. She’s defending the point of view of Britain with all the energy we know she has. And I’m doing the same on the side of the European Union.

Despite all our best efforts and the significant progress we and our teams have made over the past days on the remaining withdrawal issues, it was not possible to reach a complete agreement today.

We now have a common understanding on most relative issues – with just two or three open for discussion which require further consultation, further negotiation and further discussion.

We stand ready to resume the negotiations with the United Kingdom here in Brussels later this week. But I have to say that we were narrowing our positions to a huge extent extent today thanks to the British prime minister and thanks to the willingness of the European Commission to have a fair deal with Britain.

I’m still confident that we can reach sufficient progress before the European council on 15 December. This is not a failure, this is the start of the very last round. I am very confident that we will reach agreement in the course of this week.

Theresa May:

As President Juncker has said, we have had a constructive meeting today. Both sides have been working hard in good faith. We’ve been negotiating hard, and a lot of progress has been made.

On many of the issues there is a common understanding. And it is clear, crucially, that we want to move forward together.

But on a couple of issues some differences do remain which require further negotiation and consultation and those will continue.

But we will reconvene before the end of the week and I am also confident that we will conclude this positively.