If the UK leaves the EU trading bloc no “technical” solution can avoid the return of a physical trade border, according to the Northern Ireland Affairs Committee at Westminster. It said in a report on Friday: “We have found no evidence that, right now, an invisible border is possible.”

The cross-party committee is echoing pretty much every economic and trade expert. But, coming from inside the House of Commons, it is politically important that this has been said. And the timing is vital, ahead of a vital few days of talks before next week’s European Union summit.

The question now, in the run-up to the summit, is whether the border issue can be moved on enough to allow EU leaders to sign off on an agreement on the transition phase after Brexit. Progress on a transition is now seen as essential, particularly as businesses in both the UK and the EU need to know that things will not change suddenly after the United Kingdom leaves, next March. The transition is due to last until December 2020.

The Westminster committee, like the EU and the Irish Government, called on the British government to show how a “hard” trade border could be avoided should the UK leave the EU customs union and single market. The trouble is that everybody knows this is a logical impossibility unless the UK promises to make the customs and regulatory regimes in the North mirror those in place in the Republic. The technical difficulty of doing this will be very significant once the link with the EU trading bloc is broken. And then of course there are the political issues, including the DUP’s insistence that there be no new barriers to trade between the North and Britain.

Will this all derail an agreement next week? Or will, as some speculation suggests, a transition deal be reached, contigent on later progress on withdrawal issues, including the Border? This would effectively delay the final decision on the Border until later in the talks. A delicate dance lies ahead to see if the way can be cleared for progress at the summit, with the EU – and Ireland – having to decide how far to push for Border clarity right now and how much to leave to future negotiations also covering new trade arrangements between both sides.