The Northern Ireland 'backstop' Q&A
The Northern Ireland "backstop" has emerged as the main stumbling block in the Brexit negotiations.
What is the backstop?
The backstop is intended to be a temporary arrangement to ensure there is no return of a "hard border" between Northern Ireland and the Republic of Ireland after Britain leaves the EU in March 2019 while the two sides negotiate a free trade deal.
Why is it needed?
The EU says Britain's decision to leave the single market and the customs union means that until there is a trade deal, checks on goods crossing to and from the UK will be necessary in order to prevent exporters circumventing EU tariffs and customs controls.
So how would it work?
Under the EU plan, Northern Ireland would effectively remain part of the single market and the customs union - and subject to their rules - until a free trade agreement is in place.
However, that would mean checks on goods moving between Northern Ireland and the rest of the UK.
What is the UK's response?
Theresa May has said any arrangement which effectively draws a "border in the Irish Sea" between Northern Ireland and the rest of the UK is completely unacceptable.
Instead, she wants to keep the whole of the UK aligned with the EU while a free trade deal is negotiated.
So what is the sticking point?
The British say any such deal should be strictly time-limited - otherwise the UK could find itself tied to the EU indefinitely, with no assurance it would ever get a trade deal.
However, the EU says that would undermine the whole principle of the backstop as there would be no guarantee that there would not be a return to a hard border once the arrangement expires.
What happens now?
Downing Street has said Mrs May will never sign up to an agreement which traps the UK in a permanent customs union with the EU.
However, Tory Brexiteers fear she is preparing to give ground in an attempt to secure agreement at next week's crucial Brussels summit.