A claim by Secretary of State Brandon Lewis that there is no Irish Sea border does not match the legal reality, the High Court has heard.
The assertion was disputed as challenges to the Northern Ireland Protocol were pencilled in for a full hearing in May.
A cross-party group of unionist politicians are attempting to judicially review the UK government over the post-Brexit trading arrangements.
Senior DUP and Ulster Unionist Party representatives have backed proceedings brought by TUV leader Jim Allister.
Two further challenges are being taken: one by UUP leader Steve Aiken and party chair Lord Empey; and another by loyalist pastor Clifford Peeples.
The cases involve claims that the protocol breaches both the Act of Union and the Good Friday Agreement.
As the litigation reached court for the first time yesterday, John Larkin QC, representing Mr Allister, described it as having "undeniable constitutional importance".
He said: "This is a case in which it's a matter of some notoriety that the Secretary of State expressed the view publicly that there's no border in the Irish Sea.
"That simply doesn't correspond with the legal reality that we say undoubtedly exists."
Mr Larkin, the former Attorney General, also raised the possibility of seeking to question government officials as part of the proceedings.
"The averments in the affidavits are going to be looked at very closely indeed," he said.
"It certainly can't be ruled out at this stage that an application for cross-examination of deponents may be required."
However, Mr Justice Colton indicated his view that the focus should be on legal arguments.
"The court would be very slow to engage in what various politicians have said in the public environment," the judge said.
"This is very much, it seems to me, a dry legal analysis of constitutional points."
Amid disruption to business at Irish Sea ports, unionists are contesting the lawfulness of the protocol.
The arrangement, which formed part of the Brexit deal between the EU and UK, was aimed at preventing a hardening of the land border on the island of Ireland.
Under the terms of the protocol Northern Ireland remains in the EU single market for goods but this has o led to checks carried out on items coming into the region from the rest of the UK.
Senior counsel for the government confirmed that all three challenges will be resisted.
With overlapping legal points, Tony McGleenan QC argued that a lead case should be identified to save court time and protect the public purse.
"We oppose the progression of three cases, we don't think there should even be two cases," he said.
Mr Justice Colton told the parties he may rule later this week on whether to grant leave to seek a judicial review.
If an arguable case is established the challenges will then advance to a full hearing over four days, beginning on May 13.
"I'm attracted to the proposition that the applicants could collaborate and agree a joint position; if they are not able to do so I will decide a lead case," the judge added.
"I recognise the importance of the matter and I consider there is a requirement for expedition."
