Theresa May told parliamentarians that while a deal was under way, the Irish border remained a "point of considerable conflict."
At the EU Council summit last week, European leaders scrapped plans for a special Brexit summit in November, saying "not enough progress has been made".
Ms. May defended her offer of an extended transition period by saying that this was "undesirable" and would have to end "long before" May 2022.
But a solution on how to prevent a hard border between Northern Ireland and the Republic remains elusive, and time is running out: the UK has to leave the EU on March 29, 2019.
What is Brexit's support?
The backup is a kind of last resort, safety net or insurance policy.
The endorsement is meant to ensure that whatever happens with the rest of the negotiations, there will be no hard border between Northern Ireland and the Republic.
Therefore, even if the rest of the UK leaves the EU without trade or security agreements, there will suddenly be no border controls and restrictions on the island of Ireland.
Currently, goods and services are marketed between the two jurisdictions with few restrictions.
Given that the United Kingdom and Ireland are currently part of the EU's single market and customs union, the products do not need to be inspected for customs and standards, but after Brexit, all that could change.
Brext backing: two men dressed as customs officers take part in a protest in Belfast against Brexit
Both the EU and the United Kingdom have said they do not want a hard border and want a safety net in case all the other negotiations fall apart.
But the problem is that even back-up negotiation is impossible to achieve; So far, all that has been agreed upon is the need for support.
Why is the Northern Ireland border so important?
The Irish border is involved in many sensitive issues, which can be defined in two main areas: political and economic.
The basic pillar of peace in Northern Ireland, the Good Friday Agreement of 1998, eliminated the security checkpoints of the Irish border and made it practically invisible.
Prior to this agreement, a long period of conflict had spread since the 1920s, known in the second half of the 20th century as The Problems.
The support of Brexit: the Irish border is involved in many sensitive issues
The problems were a period of violence between two groups: the Republicans (who wanted Northern Ireland to rejoin the Republic) and the loyalists (who wanted to remain part of the United Kingdom). Many people died in the fighting.
There is concern that a return to a hard frontier could rekindle the political violence of The Troubles.
In addition to this, and creating a catch-22, there is the concern to create a hard border in the Irish Sea, cutting Northern Ireland from the rest of the United Kingdom.
Theresa May has said that this is something that "she will never accept", since "she would divide our country in two".
Brexit backstop: a solution on how to prevent a hard border in Ireland remains elusive
Essentially, if the Irish border remains fluid, there would still be checks to be made for products entering and leaving the rest of the United Kingdom and the EU.
If there is no hard border in Ireland, those goods should be verified in the ports, which means that trade between Northern Ireland and the rest of the United Kingdom will be affected.
So a hard border in Ireland will not work, and a border in the Irish Sea will not work.
And why do we need to have an edge of some kind? Because the United Kingdom has insisted on abandoning the EU's single market and the customs union.
That means that, in a way, everything that enters and leaves the UK to and from the EU must go through border control.
Backup Brexit: the port of Belfast, the main point for the transit of goods and people by sea
The other concern, of course, is economic.
The economies of Northern Ireland and the Republic are completely interconnected.
Large amounts of goods and services cross the border every day without checks of any kind.
It is estimated that at least 30,000 people travel across the border each day to work.
The Republic and Northern Ireland have a common Travel Area that prepares the EU, and both parties insist that it will remain in place in spite of everything.
But this does not prevent a hard border from arising for everything else.