The so-called Irish border backstop is one of the most controversial parts of the PM’s Brexit deal. This is what it means:
What is the backstop?
The backstop was invented to meet promises to keep open the border between Ireland and Northern Ireland even if there is no comprehensive UK-EU trade deal.
The divorce deal says it will kick in automatically at the end of the Brexit transition if that deal is not in place.
If effectively keeps the UK in a customs union with the EU and Northern Ireland in both the customs union and single market.
This means many EU laws will keep being imposed on the UK and there can be no new trade deals. It also means regulatory checks on some goods crossing the Irish Sea.
Why have Ireland and the EU demanded it?
Because Britain demanded to leave the EU customs union and single market, the EU said it needed guarantees people and goods circulating inside met EU rules.
This is covered by the Brexit transition, which effectively maintains current rules, and can in theory be done in the comprehensive EU-UK trade deal.
But the EU said there had to be a backstop to cover what happens in any gap between transition and final deal.
Why do critics hate it?
Because Britain cannot decide when to leave the backstop.
Getting out – even if there is a trade deal – can only happen if both sides agree people and goods can freely cross the border.
Brexiteers fear the EU will unreasonably demand the backstop continues so EU law continues to apply in Northern Ireland.
Northern Ireland MPs also hate the regulatory border in the Irish Sea, insisting it unreasonably carves up the United Kingdom.
What concessions did Britain get in negotiating it?
During the negotiations, Britain persuaded Brussels the backstop should apply to the whole UK and not just Northern Ireland. Importantly, this prevents a customs border down the Irish Sea – even if some goods still need to be checked.
The Government said this means Britain gets many of the benefits of EU membership after transition without all of the commitments – meaning Brussels will be eager to end the backstop.
It also got promises the EU will act in ‘good faith’ during the future trade talks and use its ‘best endeavours’ to finalise a deal – promises it says can be enforced in court.
What did the legal advice say about it?
Attorney General Geoffrey Cox said even with the EU promises, if a trade deal cannot be reached the backstop could last forever.
This would leave Britain stuck in a Brexit limbo, living under EU rules it had no say in writing and no way to unilaterally end it.