The Brexit backstop: what is it and why is it proving controversial?
Central to the Brexit negotiations was the need to maintain an open border between Northern Ireland and the Republic.
In November EU leaders approved a withdrawal deal with the UK that includes an agreement on the Irish border, involving the controversial "backstop".
Brexiteers argue that the arrangement, if ever triggered, would lock the UK into a permanent customs union with the EU, thus preventing Britain from striking lucrative new trade deals or fulfilling the promises of the 2016 referendum.
The DUP, which gives Theresa May crucial Commons support, fears the backstop would see new regulatory checks on goods travelling between Northern Ireland and the rest of the UK.
Just what is the backstop?
The backstop is effectively an insurance arrangement required by the EU and would see the UK enter into a temporary customs union with the EU if a future trade deal was not agreed during the transition period.
It 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.
Why is it controversial?
According to its critics, there are two main problems with the backstop. The first, as revealed in legal advice provided by Attorney General Geoffrey Cox to the Cabinet, is that the UK would not be able to unilaterally exit the mechanism without the EU's approval.
The second is that the wording of the backstop means technically it could be indefinite. The Withdrawal Agreement says only that the provisions apply "unless and until they are superseded by a subsequent agreement".
What does the Government say?
Mrs May has consistently said that the backstop, if it comes in at all, would only be temporary and has insisted the UK and EU will reach an agreement over future trade in the transition period. She has been seeking "further clarifications" in a bid to allay fears.
What does the EU say?
Irish Taoiseach Leo Varadkar insists there can be no changes to the backstop. He said he spoke to German chancellor Angela Merkel on Thursday and they agreed to "stand by" the Brexit deal.
The European Commission weighed in behind Mr Varadkar and confirmed "no further meetings are foreseen" with the UK on updating Mrs May's Brexit deal as negotiations have concluded.