Theresa May’s efforts to find a Plan B for Brexit which might secure parliamentary support appear to be focusing on changes to the “backstop” arrangement.

But what is the backstop and how could it be amended?

– What is the backstop?

The backstop is effectively an insurance arrangement 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 of Ireland.

Northern Ireland would also abide by EU single market rules on goods, to avoid any need for regulatory checks of products crossing the border.

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MPs rejected Theresa May’s Withdrawal Agreement (Parliament/Mark Duffy/PA)

– Why is it needed?

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 in Ireland by creating a visible symbol of division which might be targeted for attack.

– Why is it controversial?

Critics are concerned that the UK would not be able to unilaterally exit the mechanism without the EU’s approval. So long as it remained a member of the customs union, the UK would not be allowed to strike its own trade deals elsewhere in the would.

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it could be indefinite, the withdrawal agreement says only that the provisions apply “unless and until they are superseded, in whole or in part, by a subsequent agreement”.

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Attorney General Geoffrey Cox gave legal advice to the Cabinet that the backstop arrangement could in theory continue indefinitely (PA)

– Who opposes it?

Brexiteers argue that the arrangement, if ever triggered, 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 Democratic Unionist Party, which gives Theresa May crucial Commons support, fears that the backstop would see new regulatory checks introduced on goods travelling between Northern Ireland and the rest of the UK, thus placing a border down the Irish Sea.

In Scotland, conversely, there are concerns that preferential access to EU markets might give Northern Ireland an unfair competitive advantage.

– What does the Government say?

Theresa May has consistently said that the backstop, if it comes in at all, would only be temporary.

She continues to insist that she expects the UK and EU to reach an agreement over future trade within the transition period, meaning the backstop would never be activated.

And ministers say that Brussels would have an incentive to ensure the backstop is ended as soon as possible, because it is not a “comfortable” arrangement for the EU.

– What could Mrs May do to placate her critics?

The Prime Minister appears to have very little room for manoeuvre without tearing up some of the “red lines” which underpin her Brexit deal.

Downing Street made clear on Monday that the PM sees the backstop as a central issue that needs to be addressed to make her Withdrawal Agreement acceptable to Parliament.

But a spokesman dismissed suggestions that she was planning to neutralise the problem by striking a separate deal with Dublin or amending the Good Friday Agreement.

A concerted effort over Christmas to secure “assurances” from Brussels resulted in a letter from European Council president Donald Tusk and European Commission president Jean-Claude Juncker stating that the backstop would be in place only “for as long as strictly necessary” and that Brussels would “use its best endeavours” speedily to conclude a trade deal.

However, this was not enough to persuade MPs to back her deal in the Commons last week, and her hopes now appear to be resting on securing a unilateral exit mechanism or a hard end-date for the arrangement.

Brussels has indicated it is reluctant to reopen the Withdrawal Agreement, but Polish foreign minister Jacek Czaputowicz has suggested that the deadlock could be broken by setting a five-year time limit on the backstop.