The EU warned on Wednesday (4 September) that in the case of a no-deal Brexit, there will be disruption on the border between the Republic of Ireland and Northern Ireland, where the EU's new external border will lie after the UK leaves the bloc.
"All the checks will have to be carried out, it will be disruptive because the backstop would have been the only way that could have avoided disruptions," an EU official said.
One of the key objectives of the EU-UK divorce negotiations had been to avoid any friction on the border and preserve the delicate calm between the island's communities - Catholic and Protestant, unionist and nationalist - rooted in the 1998 peace deal, the Good Friday agreement.
British prime minister Boris Johnson has rejected the backstop, which requires the UK to remain in the EU's customs union and parts of the single market to ensure an open border, until a future trade deal solved the issue.
The backstop was also the reason why the British parliament rejected the withdrawal agreement three times.
On Wednesday, the commission reiterated that "the backstop provided for by the withdrawal agreement is the only solution identified that safeguards the Good Friday Agreement, ensures compliance with international law obligations and preserves the integrity of the internal market".
The EU said the UK should bring concrete alternative proposals, which British negotiators have so far failed to do, EU officials have said repeatedly.
"The UK has not submitted any structured, written, detailed alternatives, and probably it won't," another EU official said.
"The commission and Ireland continue working together, in the context of the unique situation on the island of Ireland and their twin objectives of protecting the integrity of the internal market while avoiding a hard border, to identify arrangements both for contingency solutions for the immediate aftermath of a withdrawal without an agreement and for a more stable solution for the period thereafter," it said.
"There is no doubt, only the backstop was suitable to preserve the all-island economy and the situation as we know it as today," an EU official added.
While the EU has been muted on ongoing talks with Ireland on how to police the border in case of a no-deal, the legal burden will lie with Dublin.
"EU law will require that all goods entering Ireland from the United Kingdom be subject to the relevant checks and controls to protect the safety and health of EU citizens, preserve the integrity of the internal market and enforce compliance with fiscal obligations (duties, indirect taxes)," the commission said.