Explainer: What is the 'backstop' and why does it remain a major sticking point in Brexit talks?
British prime minister insists to EU that so-called 'backstop' must be 'temporary'
British and EU officials agreed the talks ahead of tomorrow's crucial summit had been 'put on hold'
Why is Brexit back in the news?
The truth is that Brexit never really left the news agenda but it did slip off the front pages for a while as negotiations dragged on in Brussels.
We are now reaching a crunch point where big decisions will have to be made.
A meeting of EU leaders this week has long been signalled as the key deadline for making some progress, particularly on the question of the Irish Border.
If neither the EU nor UK wants a border on the island of Ireland, why is it the main sticking point in the negotiations?
Last December, both sides agreed to the so-called 'backstop' which would ensure that, regardless of whatever else happens, nothing will lead to the re-emergence of a hard border in Ireland.
However, they are split on how to achieve this. The obvious answer would be for Northern Ireland to remain inside the customs union and single market. This idea is ridiculed by the DUP, which insists that Northern Ireland must be treated the same as the rest of the UK.
What's the UK's alternative?
The UK government suggested applying a backstop customs deal to Britain too.
Brussels dismissed that as a dodge to get British goods into the single market by a back door.
Didn't a majority of people in Northern Ireland vote to stay in the EU?
Yes. Almost 56pc of voters in Northern Ireland voted to Remain. Most political parties advocated this position but the DUP backed Leave.
The fact that most people in the North want to stay in the EU isn't factored into Theresa May's thinking on Brexit.
She believes the vote of the whole of the UK is the one that matters.
So what happened at the weekend?
EU negotiators tried to take the sting out of the backstop by stressing it might never be triggered.
They said the transition could be extended by a year to December 2021 to give more time to clinch an EU-UK trade deal that would keep the Irish Border fully open anyway.
But that didn't wash?
British negotiators insisted that a permanent EU-UK customs arrangement was the answer.
It's a position EU officials see as keeping the UK in the EU customs union but using a different name because May and many of her allies have ruled that out.
So we're at an impasse?
The EU response was that such a customs union is possible but it can only be negotiated and agreed after Brexit and in any case not in the next few weeks - hence the EU demanding a legally binding backstop must remain.
The UK's Brexit Secretary, Dominic Raab, flew in to confirm the impasse with EU negotiator Michel Barnier on Sunday evening - and also raised new problems that EU negotiators thought were settled.
What happens next?
Theresa May is sounding out her allies ahead of a trip to Brussels tomorrow for summits with other EU leaders. Negotiations have been paused until after those meetings.