The Northern Ireland Protocol is "entirely lawful", with claims that it breaches the Good Friday Agreement untenable, the High Court has heard.
Counsel for the British Government insisted Parliament had full legal authority to implement the post-Brexit trading arrangements.
But a lawyer representing senior unionist politicians likened the legislative changes to "constitutional vandalism".
Judgment was reserved following three days of arguments on the protocol put in place following the UK's withdrawal from Europe.
With Northern Ireland remaining in the EU single market for goods, it has created a trade border between the region and Great Britain.
TUV leader Jim Allister and outgoing DUP and UUP chiefs Arlene Foster and Steve Aiken are among a group mounting a challenge to the arrangements.
They claim the protocol contravenes Article 6 of the Act of Union 1800 - which created a customs union between Britain and Ireland - and the 1998 Good Friday Agreement.
But according to Tony McGleenan QC, for the government, there is no legal barrier to primary legislation which Parliament endorsed as part of Brexit.
The Act of Union is not an "impediment" to provisions which amend the Northern Ireland Act, he told the court.
Part of the case centred on the concept of implied repeal, where a later Act is assumed to take precedence over earlier conflicting legislation.
Mr Justice Colton was taken through the so-called Henry VIII clause, a term for amending a law with subordinate legislation.
Counsel maintained that Parliament specifically outlined the scope for such provisions within the 2020 Withdrawal Act.
During submissions, he pointed out how the intention was to ensure north-south cooperation in Ireland.
"What was done was done appropriately to implement the protocol in the Withdrawal Agreement, and was done in a manner which was entirely compatible with the terms of the Northern Ireland Act 1998," he added.
The barrister also disputed claims that the protocol breached provisions put in place following the Good Friday Agreement.
"The idea there's a breach of the Belfast Agreement is not a tenable proposition," he said.
"It has been repeatedly established in various tiers of the courts in this jurisdiction that the Agreement per se does not have effect in domestic law. The argument that it does has been repeatedly rejected."
However, John Larkin QC, for the unionists behind the challenge, countered that nothing had been produced to indicate an intention to repeal Article 6.
Referring at one point to Australian movie The Castle, he quoted a character who uses the expression 'it's the vibe', to illustrate his point.
"My learned friend's submission in relation to implied repeal is that 'it's the vibe'," Mr Larkin said.
"He cannot point to any specific piece of legislation whether by itself, or with another piece of legislation, that effects that implied repeal.
"If there is to be constitutional vandalism, it must be deliberate and the government must own up to it at the time."
After checking on the name of the film, Mr Justice Colton remarked: "That's the weekend viewing then."
Thanking the parties for their submissions, the judge pledged: "You have given me much to consider. I will have to reserve judgment, but I will give it as soon as I possibly can."