The Brexit backstop: key questions
Theresa May's hopes of getting a Brexit deal through Parliament could hinge on finding a way of ditching the Irish backstop. But what is it and how could it be replaced?
What is the backstop?
The backstop is effectively an insurance policy required by the EU to ensure the border between Northern Ireland and the Republic remains open if no wider deal is agreed on future UK/EU trade.
It would see the UK enter into a temporary customs union with the EU if no trade deal is sealed by the end of a transition period after Brexit, which lasts until December 2020 and could be extended to the end of 2022.
The backstop would ensure there is no need for customs checks on goods travelling between Northern Ireland and the Republic.
Northern Ireland would also abide by EU single market rules on goods, to avoid any need for regulatory checks of products crossing the border.
Why is it needed?
Outside the single market and customs union, the post-Brexit UK could have significantly different tariffs and standards to the European Union.
Brussels fears that the UK's only land border with the remaining EU could become a conduit for smuggling after Brexit if there is no deal in place, allowing goods which do not meet Brussels' regulations into the 27 member states.
There are concerns on all sides that the construction of border posts and checkpoints - or even the installation of cameras - could set back the peace process by creating a visible symbol of division which might be targeted for attack.
Who opposes it?
Brexiteers fear it could lock the UK into a permanent customs union with the EU - thus preventing Britain from striking lucrative new trade deals and fulfilling the promises of the 2016 referendum.
The DUP, which gives Mrs May crucial Commons support, fears that the backstop could lead to divergence in rules between Northern Ireland and the rest of the UK - effectively creating a regulatory border in the Irish Sea.
Former Northern Ireland first minister Lord Trimble has said he is considering a judicial review, claiming the imposition of the backstop would breach the Good Friday Agreement.
What could be done to resolve the issue?
MPs registered their opposition to the backstop in a key vote on January 29, which called on the Government to find "alternative arrangements" instead.
An "alternative arrangements working group" bringing together Brexiteers and former Remainers on the Conservative benches has begun efforts to resolve the issue.
Downing Street has said potential alternatives to the plans in the Withdrawal Agreement include technological solutions, a strict time limit or a unilateral break clause allowing the UK to exit the backstop.
What does Brussels say?
In public, senior figures have insisted the backstop has to remain in place as the only "all weather" guarantee to avoid a hard border and the Withdrawal Agreement will not be reopened.
EU Brexit negotiator Michel Barnier said it is the "only operational solution".
But some on the UK side believe the EU will back down in order to avoid a no-deal Brexit.