The backstop remains the biggest obstacle for Brexit negotiations with the British Prime Minister Boris Johnson insisting the mechanism must be scrapped from the Withdrawal Agreement but the EU is refusing to remove the backstop from the Withdrawal Agreement.
What is the backstop?
The backstop is essentially an insurance policy or safety net that would prevent the reintroduction of a hard border with customs checks on the island of Ireland after Brexit.
It involves the UK staying in a form of customs union with the EU if no trade agreement is reached.
It is contained in the Withdrawal Agreement, negotiated by the EU and the UK in 2018, and is intended to protect the Good Friday Agreement and keep an open border between Northern Ireland and the Republic after Brexit.
If the UK signed the Withdrawal Agreement that is currently on the table, talks would immediately start about how trade between the UK and EU would be managed in future.
The backstop would only come into play if these talks did not come up with a future trade plan that would keep the Irish border free of checks.
In simple terms, the backstop would ensure that no matter what happens with the rest of the Brexit negotiations, there would be no hard border on the Island of Ireland.
Ironically, the mechanism that was meant to help to avoid a hard border could actually end up causing one.
What would a hard border mean?
In the event of a no-deal Brexit, Ireland would have to put in place EU laws to protect the single market and the UK would have to operate under World Trade Organisation rules.
WTO rules would mean tariffs on everything from food to electronics and fuel.
There would be a return to customs checks on goods at the border and physical infrastructure.
Why is the backstop so controversial?
The measure has hampered Brexit negotiations and was one of the main reasons why the negotiated divorce agreement was rejected by the British parliament.
The EU has refused to put a time frame on the backstop and hardline Brexiters fear it could effectively keep the UK tied to the EU indefinitely.
They believe the UK could end up in an "open-ended" customs union arrangement with the EU, despite reassurances the backstop is designed to be a temporary situation.
The border, which runs from Carlingford Lough to Lough Foyle, will become the only land border between the UK and the EU after Brexit.
The Good Friday Agreement enabled an "invisible" border between Northern Ireland and the Republic as part of the peace settlement.
As it stands, goods and services are traded across the border freely because both jurisdictions are part of the EU single market.
However, after Brexit that could all change and goods would be subject to customs checks.
A hard border could also be damaging to communities, jobs and individuals in both countries and spark renewed tension and conflict at the border.
Where do the EU and the UK now stand on the backstop issue?
British Prime Minister Boris Johnson has made ditching the backstop a red line for Brexit negotiations.
EU officials are adamant they will not reopen the Withdrawal Agreement agreed with Theresa May last year - of which the backstop forms an integral part.
Prime Minister Johnson has described the backstop as "anti-democratic" and demanded its removal from the divorce deal.
He said the UK is willing to legally commit not to erect border infrastructure and he has called for "flexible and creative" solutions.
But EU officials believe that preserving a 'level playing field' of customs and regulations after Brexit is crucial to the union, as whatever enters Ireland from Britain has free access to the rest of the bloc's single market.
Brussels needs to ensure that such products would not undermine agreed common standards or compete by price dumping. Without an agreement with Britain, the EU would insist on checks on the border.
Prime Minister Johnson has said the British government would endorse a Brexit deal without a backstop in it but he says the UK will leave the EU on October 31 regardless.
He says an alternative to the backstop must be found and last week the German Chancellor Angela Merkel gave him 30 days to do that.
The question now is will the UK Government be able to come up with an alternative to the backstop in that tight deadline - only time will tell.